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

In order to add configfs support, a usb function must be converted to use
the new interface. This patch converts the function to the new interface
and provides backward compatiblity layer, which can be removed after
all its users are converted to use the new interface.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>