ptp: ptp_clockmatrix: constify copied structure

The idtcm_caps structure is only copied into another structure,
so make it const.

The opportunity for this change was found using Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>