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

Converting f_serial to the new function interface requires converting
the f_serial's function code and its users.

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

The file is now compiled into a separate usb_f_serial.ko module.
The old function interface is provided by means of 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>