usb: gadget: dummy: do not call pullup() on udc_stop()

pullup() is already called properly by udc-core.c and
there's no need to call it from udc_stop(), in fact that
will cause issues.

Cc: stable@vger.kernel.org
Reviewed-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Felipe Balbi <balbi@ti.com>