staging: rtl8188eu: fix double unlock error in rtw_resume_process()

Fix following static checker warning:
    drivers/staging/rtl8188eu/os_dep/usb_intf.c:311 rtw_resume_process()
    error: double unlock 'mutex:&pwrpriv->mutex_lock'

Fixes: eaf47b71 ("staging: rtl8188eu: fix missing unlock on error in rtw_resume_process()")
Reported-By: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Wei Yongjun <weiyj.lk@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>