usb: gadget: add max_speed to usb_composite_driver

This field is used by the Gadget drivers to specify
the maximum speed they support, meaning: the maximum
speed they can provide descriptors for.

The driver speed will be set in consideration of this
value.

[ balbi@ti.com : dropped the ifdeffery ]

Signed-off-by: Tatyana Brokhman <tlinder@codeaurora.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>