net/mlx5: Move crosststamp setup into helper function

net/mlx5: Move crosststamp setup into helper function

Move the crosststamp registration logic into a dedicated helper,
mlx5_init_crosststamp().

This prepares the code for a follow-up patch around PTM handling.

Signed-off-by: Carolina Jubran <cjubran@nvidia.com>
Reviewed-by: Shahar Shitrit <shshitrit@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260316133607.8738-2-tariqt@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>