realtek: rtl931x: add missing function reference for l2_hash_seed

realtek: rtl931x: add missing function reference for l2_hash_seed

Add missing function reference for the l2_hash_seed call in rtl931x_reg
in the rtl83xx DSA driver part.
While at it, rename the referenced function to proper naming convention
and simplify its content.

The missing reference causes a hard crash after a short time (on
MS510TXM) because the driver assumes the reference always exists.

[  111.785559] CPU 0 Unable to handle kernel paging request at virtual address 00000000, epc == 00000000, ra == 805469a0
[  111.800991] Oops[#1]:
[  111.801026] CPU: 0 PID: 11 Comm: kworker/u8:0 Tainted: G           O       6.12.33 #0
[  111.801046] Workqueue: dsa_ordered dsa_slave_switchdev_event_work
...
[  111.880600] epc   : 00000000 0x0
[  111.884219] ra    : 805469a0 rtl83xx_port_fdb_add+0x7c/0x204
[  111.890570] Status: 11000403 KERNEL EXL IE
[  111.895263] Cause : 50800008 (ExcCode 02)
[  111.899731] BadVA : 00000000
[  111.902946] PrId  : 0001a120 (MIPS interAptiv (multi))
[  111.956086] Process kworker/u8:0 (pid: 11, threadinfo=0b107c25, task=265aeb31, tls=00000000)
...
[  112.015167] Call Trace:
[  112.019549] [<80170b04>] load_balance+0x494/0x708
[  112.025022] [<807bb368>] dsa_port_do_fdb_add+0x24c/0x340
[  112.031048] [<807bc868>] dsa_switch_event+0xd44/0x13cc
[  112.036845] [<8015867c>] raw_notifier_call_chain+0x48/0x88
[  112.043031] [<807bcf3c>] dsa_tree_notify+0x10/0x3c
[  112.048395] [<807b2a64>] dsa_port_bridge_host_fdb_add+0x15c/0x190
[  112.055459] [<807b4e40>] dsa_slave_switchdev_event_work+0x164/0x1cc
...

Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/19170
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>