usb: gadget: f_uac1: convert to new function interface with backward compatibility

Converting uac1 to the new function interface requires converting
the USB uac1's function code and its users.

This patch converts the f_uac1.c to the new function interface.

The file is now compiled into a separate usb_f_uac1.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.

Tested-by: Sebastian Reimers <sebastian.reimers@googlemail.com>
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>