lan966x: Fix sleeping in atomic context

lan966x: Fix sleeping in atomic context

The following warning was seen when we try to connect using ssh to the device.

BUG: sleeping function called from invalid context at kernel/locking/mutex.c:575
in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 104, name: dropbear
preempt_count: 1, expected: 0
INFO: lockdep is turned off.
CPU: 0 UID: 0 PID: 104 Comm: dropbear Tainted: G        W           6.18.0-rc2-00399-g6f1ab1b109b9-dirty #530 NONE
Tainted: [W]=WARN
Hardware name: Generic DT based system
Call trace:
 unwind_backtrace from show_stack+0x10/0x14
 show_stack from dump_stack_lvl+0x7c/0xac
 dump_stack_lvl from __might_resched+0x16c/0x2b0
 __might_resched from __mutex_lock+0x64/0xd34
 __mutex_lock from mutex_lock_nested+0x1c/0x24
 mutex_lock_nested from lan966x_stats_get+0x5c/0x558
 lan966x_stats_get from dev_get_stats+0x40/0x43c
 dev_get_stats from dev_seq_printf_stats+0x3c/0x184
 dev_seq_printf_stats from dev_seq_show+0x10/0x30
 dev_seq_show from seq_read_iter+0x350/0x4ec
 seq_read_iter from seq_read+0xfc/0x194
 seq_read from proc_reg_read+0xac/0x100
 proc_reg_read from vfs_read+0xb0/0x2b0
 vfs_read from ksys_read+0x6c/0xec
 ksys_read from ret_fast_syscall+0x0/0x1c
Exception stack(0xf0b11fa8 to 0xf0b11ff0)
1fa0:                   00000001 00001000 00000008 be9048d8 00001000 00000001
1fc0: 00000001 00001000 00000008 00000003 be905920 0000001e 00000000 00000001
1fe0: 0005404c be9048c0 00018684 b6ec2cd8

It seems that we are using a mutex in a atomic context which is wrong.
Change the mutex with a spinlock.

Fixes: 12c2d0a5b8e2 ("net: lan966x: add ethtool configuration and statistics")
Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20251105074955.1766792-1-horatiu.vultur@microchip.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>