rtlwifi: rtl8723be: Fix potential race condition

Flag rfchange_inprogress in struct rtl_ps_ctl is protected by a spinlock
in most routines but not in rtl8723be_dm_watchdog(), which could
lead to a race condition. The necessary locking to prevent this condition
is added.

Reported-by: Pavel Andrianov <andrianov@ispras.ru>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Pavel Andrianov <andrianov@ispras.ru>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>