net: phylink: fix NULL pointer deref in phylink_major_config()

net: phylink: fix NULL pointer deref in phylink_major_config()

When a MAC driver returns a PCS for an interface mode, and then we
attempt to switch to a different mode that doesn't require a PCS,
this causes phylink to oops:

Unable to handle kernel NULL pointer dereference at virtual address 0000000000000010
Mem abort info:
  ESR = 0x0000000096000044
  EC = 0x25: DABT (current EL), IL = 32 bits
  SET = 0, FnV = 0
  EA = 0, S1PTW = 0
  FSC = 0x04: level 0 translation fault
Data abort info:
  ISV = 0, ISS = 0x00000044, ISS2 = 0x00000000
  CM = 0, WnR = 1, TnD = 0, TagAccess = 0
  GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
user pgtable: 4k pages, 48-bit VAs, pgdp=0000000137f96000
[0000000000000010] pgd=0000000000000000, p4d=0000000000000000
Internal error: Oops: 0000000096000044 [#1]  SMP
Modules linked in: --
CPU: 1 UID: 0 PID: 55 Comm: kworker/u33:0 Not tainted 6.19.0-rc5-00581-g73cb8467a63e #1 PREEMPT
Hardware name: Qualcomm Technologies, Inc. Lemans Ride Rev3 (DT)
Workqueue: events_power_efficient phylink_resolve
pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS    +BTYPE=--)
pc : phylink_major_config+0x408/0x948
lr : phylink_major_config+0x3fc/0x948
sp : ffff800080353c60
x29: ffff800080353cb0 x28: ffffb305068a8a00 x27: ffffb305068a8000
x26: ffff000080092100 x25: 0000000000000000 x24: 0000000000000000
x23: 0000000000000001 x22: 0000000000000000 x21: ffffb3050555b3d0
x20: ffff800080353d10 x19: ffff0000b6059400 x18: 00000000ffffffff
x17: 74756f2f79687020 x16: ffffb305045e4f18 x15: 6769666e6f632072
x14: 6f6a616d203a3168 x13: 782d657361623030 x12: ffffb305068c6a98
x11: 0000000000000583 x10: 0000000000000018 x9 : ffffb305068c6a98
x8 : 0000000100006583 x7 : 0000000000000000 x6 : ffff00008083cc40
x5 : ffff00008083cc40 x4 : 0000000000000001 x3 : 0000000000000001
x2 : 0000000000000000 x1 : 0000000000000000 x0 : ffff0000b269e5a8
Call trace:
 phylink_major_config+0x408/0x948 (P)
 phylink_resolve+0x294/0x6e4
 process_one_work+0x148/0x28c
 worker_thread+0x2d8/0x3d8
 kthread+0x134/0x208
 ret_from_fork+0x10/0x20
Code: d63f0020 f9400e60 b4000040 f900081f (f9000ad3)
---[ end trace 0000000000000000 ]---

This is caused by "pcs" being NULL when we attempt to execute:

		pcs->phylink = pl;

Make this conditional on pcs being non-null.

Fixes: 486dc391ef43 ("net: phylink: allow mac_select_pcs() to remove a PCS")
Reported-by: Mohd Ayaan Anwar <mohd.anwar@oss.qualcomm.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1vl39Q-00000006utm-229h@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>