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>