staging: rtl8712: fix double lock bug in SetPSModeWorkItemCallback()

Fix a double lock bug in SetPSModeWorkItemCallback().

Fixes: 5c2ba8b8

 ("rtl8712: pwrctrl_priv: Replace semaphore lock with mutex")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>