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

This patch removes string registration from the Mass Storage
Gadget.  With recent changes to the composite framework, all
that we need is handled by the composite layer.  This means
composite registers a string ID for manufacturer and product.

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>