usb: gadget: fsl_qe_udc: signedness bug in qe_get_frame()

We can't assign -EINVAL to a u16.

Fixes: 3948f0e0 ('usb: add Freescale QE/CPM USB peripheral controller driver')
Acked-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>