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

Converting ecm subset to the new function interface requires converting
the USB subset's function code and its users.

This patch converts the f_subset.c to the new function interface.

The file is now compiled into a separate usb_f_subset.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>
Signed-off-by: Felipe Balbi <balbi@ti.com>