USB: gadget: composite: usb_string_ids_*() functions added

usb_string_ids_tab() and usb_string_ids_n() functions added to
the composite framework.  The first accepts an array of
usb_string object and for each registeres a string id and the
second registeres a given number of ids and returns the first.

This may simplify string ids registration since gadgets and
composite functions won't have to call usb_string_id() several
times and each time check for errer status -- all this will be
done with a single call.

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>