usb: gadget: fsl: fix null pointer checking

fsl_ep_fifo_status() should return error if _ep->desc is null.

Fixes: 75eaa498 (“usb: gadget: Correct NULL pointer checking in fsl gadget”)
Reviewed-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Ran Wang <ran.wang_1@nxp.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>