net/mlx5e: Use macro for TC header re-write offload field mapping

Use a macro for the static mapping between the enumeration of field
supported by the firmware for header re-write to the corresponding
network header field. This improves the readability of the code and
doesn't change any functionality.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Reviewed-by: Paul Blakey <paulb@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>