ptp: clockmatrix: Fix non-zero phase_adj is lost after snap

Fix non-zero phase_adj is lost after snap. Use ktime_sub
to do ktime_t subtraction.

Signed-off-by: Min Li <min.li.xe@renesas.com>
Link: https://lore.kernel.org/r/1607442117-13661-3-git-send-email-min.li.xe@renesas.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>