net/mlx5: Extract MTCTR register read logic into helper function

Refactor the MTCTR register reading logic into a dedicated helper to
lay the groundwork for the next patch.

Signed-off-by: Carolina Jubran <cjubran@nvidia.com>
Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://patch.msgid.link/1755008228-88881-3-git-send-email-tariqt@nvidia.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>