net/mlx5: Imply MLXFW in mlx5_core

mlxfw can be compiled as external module while mlx5_core can be
builtin, in such case mlx5 will act like mlxfw is disabled.

Since mlxfw is just a service library for mlx* drivers,
imply it in mlx5_core to make it always reachable if it was enabled.

Fixes: 3ffaabec ("net/mlx5e: Support the flash device ethtool callback")
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>