usb: gadget: acm_ms: need to set max_speed

Failing to set max_speed prevents g_acm_ms working with many drivers which
check for driver->max_speed < USB_SPEED_FULL, including pxa25x_udc

Signed-off-by: Steve Bennett <steveb@workware.net.au>
Signed-off-by: Felipe Balbi <balbi@ti.com>