net/mlx5e: Move modify tirs hash functionality

Move modify tirs hash functionality (mlx5e_modify_tirs_hash) from
en_ethtool.c to en_main.c. This allows future use of this fuctionality
from en_fs_ethtool.c, while keeping current convention: en_ethtool.c
doesn't have an API.  There is no functional change here.

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