USB: composite: Fix bug: low byte of w_index is the usb interface number not the whole 2 bytes of w_index

In some usb gadget driver, for example usb audio class device, the high
byte of w_index is the entity id and low byte is the interface number.
If we use the 2 bytes of w_index as the array number, we will get a
wrong pointer or NULL pointer.

This patch fixes this issue.
Signed-off-by: Bryan Wu <cooloney@kernel.org>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>