usb: gadget: convert source sink and loopback to new function interface

This patch converts the f_sourcesink and f_loopback file to the USB-function
module. Both functions shares a few common utility functions which are
currently implemented in g_zero.c itself. This patch moves the common
code into the sourcesink file and creates one module out of the the two
functions (source sink and loop back).
The g_zero gadget is function specific to source sink and loop back to
set a few options. This Symbol dependency enforces a modul load right
now.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>