rtl8367b: fix RTL8367S-VB vlan mc memory handling

rtl8367b: fix RTL8367S-VB vlan mc memory handling

The RTL8367S-VB (Family D) uses a software-emulated VLAN MC table
(emu_vlanmc) since it lacks hardware support for the VLAN MC registers
used by earlier chips.

- Add missing NULL check after kzalloc() in rtl8367b_init_regs() to
  prevent NULL pointer dereference on allocation failure.
- Add missing kfree(smi->emu_vlanmc) in rtl8367b_remove() to prevent
  memory leak on driver unload.

Signed-off-by: Mieczyslaw Nalewaj <namiltd@yahoo.com>
Link: https://github.com/openwrt/openwrt/pull/23970
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>