usb: mv_udc: Clean up the initial variable check

Clean up the code that checks the validity of a USB gadget driver
in usb_gadget_register_driver(). Moreover, limit the speed of the
driver to either FULL or HIGH, this is more precise and once we
have xHCI support, also more correct.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Lei Wen <leiwen@marvell.com>
Cc: Otavio Salvador <otavio@ossystems.com.br>
Cc: Stefano Babic <sbabic@denx.de>