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>
diff --git a/drivers/usb/gadget/Kconfig b/drivers/usb/gadget/Kconfig
index 8aefbbd..b61c72f 100644
--- a/drivers/usb/gadget/Kconfig
+++ b/drivers/usb/gadget/Kconfig
@@ -500,6 +500,9 @@
 	tristate
 	depends on USB_GADGET
 
+config USB_F_ACM
+	tristate
+
 config USB_F_SS_LB
 	tristate
 
@@ -758,6 +761,7 @@
 config USB_G_SERIAL
 	tristate "Serial Gadget (with CDC ACM and CDC OBEX support)"
 	select USB_U_SERIAL
+	select USB_F_ACM
 	select USB_LIBCOMPOSITE
 	help
 	  The Serial Gadget talks to the Linux-USB generic serial driver.