wifi: rtw88: add mutex when set SAR

Applying SAR will access hal data, it should hold rtwdev::mutex
to avoid hal data changed during setting flow.

Signed-off-by: Chih-Kang Chang <gary.chang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220809084107.38137-2-pkshih@realtek.com