usb: chipidea: drop redundant NULL check

Currently, gadget can't be NULL in _gadget_stop_activity().

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Reported-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>