usb/gadget: use generic usb_gadget_poll() in u_serial
Use the generic gadget polling function instead of the
specific fsl function. This is a fix from a leftover in
gadget development.
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>