USB: imx_udc: fix leak in imx_ep_alloc_request()

cppcheck found another leak in drivers/usb/gadget/imx_udc.c

Cc: Mike Lee <eemike@gmail.com>
Cc: Darius Augulis <augulis.darius@gmail.com>
Signed-off-by: Daniel Mack <daniel@caiaq.de>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>