usb: gadget: f_acm: convert to new function interface with backwards compatibility

This patch converts f_acm into a module which uses the new function
interface. It also converts one of its users that is g_serial to make
use of it. The other users of it (g_nokia for instance) are still using
the old include file system and should not notice the change at all. So
they can be converter later independently.

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

drivers/usb/gadget/acm_ms.c

@@ -40,7 +40,7 @@
  * the runtime footprint, and giving us at least some parts of what
  * a "gcc --combine ... part1.c part2.c part3.c ... " build would.
  */
-
+#define USB_FACM_INCLUDED
 #include "f_acm.c"
 #include "f_mass_storage.c"