usb: gadget: dummy_hcd: use gadget_to_dummy_hcd() where possible

gadget_to_dummy_hcd() already does the speed check, so
it's unnecessary to unroll that all the time.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>