usb: gadget: printer: call usb_add_function() last

Conversion to the new function interface requires splitting a
<something>_bind_config() function into two parts: allocation of
container_of struct usb_function and invocation of usb_add_function().
This patch moves the latter to the end of the f_printer_bind_config()
in order to enable conversion to the new interface.

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