usb: gadget: remove unused shortname variable

The shortname variable isn't referenced anywhere in the code, so just
remove it.

Pointed out by a GCC 6.2 default warning option.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Acked-by: Lukasz Majewski <l.majewski@samsung.com>