usb: gadget: configfs: Add max_speed setting

Some functions support speeds other than SuperSpeed. Add max_speed
attribute to configfs gadget allowing user to specify the maximum speed
the composite driver supports. The valid input speed names are
super-speed-plus, super-speed, high-speed, full-speed, and low-speed.

Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>