net/mlx5: Fix ptp max frequency adjustment range

.max_adj of ptp_clock_info acts as an absolute value for the amount in ppb
that can be set for a single call of .adjfine. This means that a single
call to .getfine cannot be greater than .max_adj or less than -(.max_adj).
Provides correct value for max frequency adjustment value supported by
devices.

Fixes: 3d8c38af ("net/mlx5e: Add PTP Hardware Clock (PHC) support")
Signed-off-by: Rahul Rameshbabu <rrameshbabu@nvidia.com>
Reviewed-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>