usb: dwc3: gadget: Remove invalid low-speed setting

None of the DWC_usb3x IPs (and all their versions) supports low-speed
setting in device mode. In the early days, our "Early Adopter Edition"
DWC_usb3 databook shows that the controller may be configured to operate
in low-speed, but it was revised on release. Let's remove this invalid
speed setting to avoid any confusion.

Signed-off-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Link: https://lore.kernel.org/r/258b1c7fbb966454f4c4c2c1367508998498fc30.1615509438.git.Thinh.Nguyen@synopsys.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>