usb: gadget: f_ecm: convert to new function interface with backward compatibility

Converting ecm to the new function interface requires converting
the USB ecm's function code and its users.

This patch converts the f_ecm.c to the new function interface.

The file is now compiled into a separate usb_f_ecm.ko module.

The old function interface is provided by means of a preprocessor
conditional directives. After all users are converted, the old interface
can be removed.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
diff --git a/drivers/usb/gadget/g_ffs.c b/drivers/usb/gadget/g_ffs.c
index fbfdb53..d38a073 100644
--- a/drivers/usb/gadget/g_ffs.c
+++ b/drivers/usb/gadget/g_ffs.c
@@ -28,6 +28,7 @@
 #    define USB_ETH_RNDIS y
 #  endif
 
+#define USBF_ECM_INCLUDED
 #  include "f_ecm.c"
 #  include "f_subset.c"
 #  ifdef USB_ETH_RNDIS