wifi: rtl8xxxu: make instances of iface limit and combination to be static const

rtl8xxxu_limits and rtl8xxxu_combinations can be static const, so add
modifiers as desire. Otherwise, Sparse reports warnings

rtl8xxxu_core.c:7677:30: warning: symbol 'rtl8xxxu_limits' was not declared. Should it be static?
rtl8xxxu_core.c:7682:36: warning: symbol 'rtl8xxxu_combinations' was not declared. Should it be static?

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/20240116080945.20172-2-pkshih@realtek.com