usb: gadget: storage_common: pass filesem to fsg_store_cdrom

If cdrom flag is set ro flag is implied. Try setting the ro first, and
only if it succeeds set the cdrom flag.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>