net/mlx5e: Remove mlx5e_dbg() and msglvl support

The msglvl support was implemented using the mlx5e_dbg() macro which is
rarely used in the driver, and is not very useful when you can just use
dynamic debug instead.
Remove mlx5e_dbg() and convert its usages to netdev_dbg().

Signed-off-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>