usb: gadget: Correct NULL pointer checking in fsl gadget

Correct NULL pointer checking for endpoint descriptor
before it gets dereferenced

Signed-off-by: Nikhil Badola <nikhil.badola@freescale.com>
Signed-off-by: Ran Wang <ran.wang_1@nxp.com>
Reviewed-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>