mx6memcal: Disable USB GADGET in SPL

As this board does not use CONFIG_OF_CONTROL and the DM_USB migration
deadline has passed, disable USB_GADGET support.

Cc: Eric Nelson <eric@nelint.com>
Cc: Stefano Babic <sbabic@denx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Eric Nelson <eric@nelint.com>