net/mlx5e: Improve ethtool private-flags code structure

Refactor the code of private-flags setter.
Replace consecutive calls to mlx5e_handle_pflag with a loop
that uses a preset set of parameters.

Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>