wifi: rtw88: simplify vif iterators

Since all iterators called by 'rtw_iterate_vifs()' never uses
'mac' argument, it may be omitted, and 'struct rtw_vifs_entry'
may be simplified accordingly.

Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Reviewed-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230628072327.167196-4-dmantipov@yandex.ru