usb: dwc3: gadget: Set gadget_max_speed when set ssp_rate

usb: dwc3: gadget: Set gadget_max_speed when set ssp_rate

Set the dwc->gadget_max_speed to SuperSpeed Plus if the user sets the
ssp_rate. The udc_set_ssp_rate() is intended for setting the gadget's
speed to SuperSpeed Plus at the specified rate.

Fixes: b61480086100 ("usb: dwc3: gadget: Implement setting of SSP rate")
Cc: <stable@vger.kernel.org>
Signed-off-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Link: https://lore.kernel.org/r/0b2732e2f380d9912ee87f39dc82c2139223bad9.1615254129.git.Thinh.Nguyen@synopsys.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>