usb: gadget: f_mass_storage: fix default product name

If cfg->product name is not set, a default name is chosen depending
on the common->luns->cdrom flag. If the flag is set the name should
be "File-CD Gadget", and if the flag is not set the name should be
"File-Stor Gadget".

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