usb: gadget: composite: avoid access beyond array max length

One of the USB CV MSC tests issues Get Max LUN request with
invalid wIndex (wIndex = 65535) parameter.

Add proper handling to prevent array index out of bounds issue.

Signed-off-by: Maulik Mankad <maulik@ti.com>
Cc: David Brownell <david-b@pacbell.net>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>