usb: gadget: storage: Remove reference counting

The kref used to be needed because sharing of fsg_common among multiple USB
function instances was handled by fsg. Now this is managed by configfs, we
don't need it anymore. So let's eliminate kref from this driver.

Acked-by: Alan Stern <stern@rowland.harvard.edu>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Jaejoong Kim <climbbb.kim@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>