staging: rtl8712: add error handler in r8712_usbctrl_vendorreq()

When 'status' returned from usb_control_msg() is not equal to 'len',
that usb_control_msg() is on partial failure, r8712_usbctrl_vendorreq()
will treat partial reads as success.

Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Wang Cheng <wanngchenng@gmail.com>
Link: https://lore.kernel.org/r/e33ea53d36c422fbe7eabec5bd9eecb0ebce1bc5.1652618244.git.wanngchenng@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>