USB: gadget: g_multi: code clean up and refactoring

The Multifunction Composite Gadget have been cleaned up
and refactored so hopefully it looks prettier and works
at least as good as before changes.

A Kconfig has also been fixed to make it impossible to build
FunctionFS gadget with no configurations.  With this patch, if
RNDIS is not chosen by the user CDC is force-selected.

Signed-off-by: Michal Nazarewicz <m.nazarewicz@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/gadget/Kconfig

@@ -863,6 +863,7 @@ config USB_G_NOKIA
 config USB_G_MULTI
 	tristate "Multifunction Composite Gadget (EXPERIMENTAL)"
 	depends on BLOCK && NET
+	select USB_G_MULTI_CDC if !USB_G_MULTI_RNDIS
 	help
 	  The Multifunction Composite Gadget provides Ethernet (RNDIS
 	  and/or CDC Ethernet), mass storage and ACM serial link