usb: gadget: configfs: Fix set but not used variable warning

Fix a -Wunused-but-set-variable warning in gadget_string_s_store()

Fixes: 15a7cf8c ("usb: gadget: configfs: Support arbitrary string descriptors")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Daniel Scally <dan.scally@ideasonboard.com>
Link: https://lore.kernel.org/r/20230209094359.1549629-1-dan.scally@ideasonboard.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>