ptp: ocp: use snprintf() in ptp_ocp_verify()

This code is fine, but it's easier to review if we use snprintf()
instead of sprintf().

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Link: https://lore.kernel.org/r/20220318074723.GA6617@kili
Signed-off-by: Jakub Kicinski <kuba@kernel.org>