Feed kstrtou8 with NULL terminated string.

Changes since v1:
-Use sscanf to get rid of adhoc string parse.
Changes since v2:
-Check if sscanf returns 3.

Fixes: 7ea5fda2 ("ptp: ptp_clockmatrix: update to support 4.8.7 firmware")
Signed-off-by: Min Li <min.li.xe@renesas.com>
Link: https://lore.kernel.org/r/1606273115-25792-1-git-send-email-min.li.xe@renesas.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>