USB: gadget: g_multi: moved strings handling code to composite

This patch removes some of the string registration from the
Multifunction Composite Gadget as composite layer can handle
the iManufacturer and iProduct for us.

This also adds the "needs_serial" so that composite layer will
issue a warning if user space fails to provide the iSerialNumber
module parameter.

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>