usb: gadget: dummy_hcd: use dummy_pullup() instead of open coding

The removed code does the same thing as dummy_pullup(). The only
difference is that in dummy_udc_stop() the first dummy_pullup()
did not call usb_hcd_poll_rh_status().

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>