usb: dwc3: gadget: properly check ep cmd

The cmd argument we pass to
dwc3_send_gadget_ep_cmd() could contain extra
arguments embedded. When checking for StartTransfer
command, we need to make sure to match only lower 4
bits which contain the actual command and ignore the
rest.

Reported-by: Janusz Dziedzic <januszx.dziedzic@intel.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>