wifi: rtl8xxxu: support setting mac address register for both interfaces

To prepare for concurrent mode, enhance rtl8xxxu_set_mac() to write the
mac address of the respective interface to REG_MACID or REG_MACID1.

Remove the call to rtl8xxxu_set_mac() from the init function as we set
it in rtl8xxxu_add_interface() later anyway.

Until rtl8xxxu_add_interface() can handle both interfaces, call
rtl8xxxu_set_mac() with port_num = 0.

Signed-off-by: Martin Kaistra <martin.kaistra@linutronix.de>
Reviewed-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/20231222101442.626837-6-martin.kaistra@linutronix.de