usb: gadget: Do not call board_usb_xxx() directly in USB gadget drivers

Add 2 functions to wrap the calls to board_usb_init() and
board_usb_cleanup().
This is a preparatory work for DM support for UDC drivers (DM_USB_GADGET).

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>