USB: gadget: file_storage: serial parameter even if not test mode

Moved the serial parameter handling code out of "#ifdef
CONFIG_USB_FILE_STORAGE_TEST".

This modifies Yann Cantin's commit "USB: Add a serial number
parameter to g_file_storage" module as per Alan Stern's request.

Signed-off-by: Michal Nazarewicz <m.nazarewicz@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Tested-by: Anand Gadiyar <gadiyar@ti.com>
Cc: David Brownell <david-b@pacbell.net>
Cc: Yann Cantin <yann.cantin@laposte.net>