usb: gadget: fsl_qe_udc: constify qe_ep0_desc

qe_ep0_desc is only passed as the second argument to qe_ep_init, which is
const, so qe_ep0_desc can be const too.

Done with the help of Coccinelle.

Acked-by: Li Yang <leoyang.li@nxp.com>
Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>