usb: gadget: rename usb_gadget_driver::speed to max_speed

This commit renames the “speed” field of the usb_gadget_driver
structure to “max_speed”.  This is so that to make it more
apparent that the field represents the maximum speed gadget
driver can support.

This also make the field look more like fields with the same
name in usb_gadget and usb_composite_driver structures.  All
of those represent the *maximal* speed given entity supports.

After this commit, there are the following fields in various
structures:
* usb_gadget::speed - the current connection speed,
* usb_gadget::max_speed - maximal speed UDC supports,
* usb_gadget_driver::max_speed - maximal speed gadget driver
  supports, and
* usb_composite_driver::max_speed - maximal speed composite
  gadget supports.

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>