net/mlx5: Move alloc/dealloc encap commands declarations to common header file

The alloc and dealloc encap commands will be used in the mlx5e driver,
as such, declare them in a common header file.

Also, rename the functions: mlx5_cmd_{de}alloc_encap is replaced with
mlx5_encap_{de}alloc.

Signed-off-by: Hadar Hen Zion <hadarh@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>