net/mlx5: Force round second at 1PPS out start time

Allow configuration of 1PPS start time only with time-stamp representing
a round second. Prior to this patch driver allowed setting of a
non-round-second which is not supported by the device. Avoid unexpected
behavior by restricting start-time configuration to a round-second.

Fixes: 4272f9b8 ("net/mlx5e: Change 1PPS out scheme")
Signed-off-by: Aya Levin <ayal@nvidia.com>
Reviewed-by: Eran Ben Elisha <eranbe@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>