Collision of writing rf registers could occur if the driver writes
rf registers by direct write while the hardware is writing other rf
registers by pi write simultaneously.

Hardware pi write can be triggered by rf calibrations sometimes, so
the driver can not always write rf registers by direct write
protection. Direct write protection can make sure that there is no
hardware pi write during the direct write.

According to some experiments, if we add direct write protection
when switching channel, the performance of rf calibration will not
be affected.
Signed-off-by: Chien-Hsun Liao <ben.liao@realtek.com>
Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>