wifi: rtl8xxxu: Add rtl8xxxu_write{8,16,32}_{set,clear}

Also add rtl8xxxu_write32_mask, rtl8xxxu_write_rfreg_mask.

These helper functions make it easier to modify only parts of a register
by eliminating the call to the register reading function and the bit
manipulations.

Signed-off-by: Bitterblue Smith <rtl8821cerfe2@gmail.com>
Reviewed-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/9430b841-1048-b27c-14ec-fca447dc32af@gmail.com