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

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

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

The file is now compiled into a separate usb_f_rndis.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/Kconfig b/drivers/usb/gadget/Kconfig
index 3a72b9d..22c8608 100644
--- a/drivers/usb/gadget/Kconfig
+++ b/drivers/usb/gadget/Kconfig
@@ -526,6 +526,9 @@
 config USB_F_SUBSET
 	tristate
 
+config USB_F_RNDIS
+	tristate
+
 choice
 	tristate "USB Gadget Drivers"
 	default USB_ETH