usb: gadget: storage_common: use kstrto*()

This commit replaces the usage of strict_strtoul() (which
became deprecated after commit 33ee3b2e) with kstrtouint().

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