usb: dwc3: gadget: let us set lower max_speed

In some SoCs, dwc3 is implemented as a USB2.0 only
core, meaning that it can't ever achieve SuperSpeed.

Currect driver always sets gadget.max_speed to
USB_SPEED_SUPER unconditionally. This can causes
issues to some Host stacks where the host will issue
a GetBOS() request and we will reply with a BOS
containing Superspeed Capability Descriptor.

At least Windows seems to be upset by this fact and
prints a warning that we should connect $this device
to another port.

[ balbi@ti.com : rewrote entire commit, including
source code comment to make a lot clearer what the
problem is ]

Cc: <stable@vger.kernel.org>
Signed-off-by: Ben McCauley <ben.mccauley@garmin.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>