wifi: rtw88: Drop rf_lock

The rtwdev->rf_lock spinlock protects the rf register accesses in
rtw_read_rf() and rtw_write_rf(). Most callers of these functions hold
rtwdev->mutex already with the exception of the callsites in the debugfs
code. The debugfs code doesn't justify an extra lock, so acquire the mutex
there as well before calling rf register accessors and drop the now
unnecessary spinlock.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20221202081224.2779981-4-s.hauer@pengutronix.de