usb: gadget: f_mass_storage: convert to new function interface with backward compatibility

Converting mass storage to the new function interface requires converting
the USB mass storage's function code and its users.
This patch converts the f_mass_storage.c to the new function interface.
The file is now compiled into a separate usb_f_mass_storage.ko module.
The old function interface is provided by means of a preprocessor conditional
directives. After all users are converted, the old interface can be removed.

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>