usbnet: ipheth: fix DPE OoB read

commit ee591f2b upstream.

Fix an out-of-bounds DPE read, limit the number of processed DPEs to
the amount that fits into the fixed-size NDP16 header.

Fixes: a2d274c6 ("usbnet: ipheth: add CDC NCM support")
Cc: stable@vger.kernel.org
Signed-off-by: Foster Snowhill <forst@pen.gy>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>