usb: gadget: legacy: set max_speed to super-speed

These interfaces do support super-speed so let's not
limit maximum speed to high-speed.

Cc: <stable@vger.kernel.org>
Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>