net/mlx5: Revise gre and nvgre key formats

GRE RFC defines a 32 bit key field. NVGRE RFC splits the 32 bit
key field to 24 bit VSID (gre_key_h) and 8 bit flow entropy (gre_key_l).

Define the two key parsing alternatives in a union, thus enabling both
access methods.

Signed-off-by: Oz Shlomo <ozsh@mellanox.com>
Reviewed-by: Eli Britstein <elibr@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>