USB: gadget: validate endpoint index for xilinx udc

Assure that host may not manipulate the index to point
past endpoint array.

Signed-off-by: Szymon Heidrich <szymon.heidrich@gmail.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>