usb: gadget: storage_common: use kstrto*() [bug fix]

This commit fixes an embarrassing bug in the "storage_common:
use kstrto*()" patch which caused fsg_store_ro() to return
zero instead of the length of the consumed buffer.

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>