usb: gadget: f_mass_storage: If 'ro'/'cdrom' specified, open file as read-only

If we don't need Write access then attempt to open backing file in Read Only
mode instead of bailing out too soon.

Signed-off-by: Roger Quadros <roger.quadros@nokia.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>