usb: gadget: printer: factor out f_printer

The legacy printer gadget now contains both a reusable printer function
and legacy gadget proper implementations interwoven, but logically
separate. This patch factors out a reusable f_printer.
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>