usb: dwc2: gadget: implement udc_set_speed()

Implemented udc_set_speed() gadget ops to allow the udc to select
the gadget speed on initialization.

Acked-by: Minas Harutyunyan <Minas.Harutyunyan@synopsys.com>
Signed-off-by: Argishti Aleksanyan <aleksan@synopsys.com>
Link: https://lore.kernel.org/r/c453469d618100321c876a8c2b0ebee15a456eac.1628583235.git.aleksan@synopsys.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>