net/mlx5: Use fte status to decide on firmware command

An fte status becomes FS_FTE_STATUS_EXISTING only after it was
created in HW. We can use this in order to simplify the logic on
what firmware command to use. If the status isn't FS_FTE_STATUS_EXISTING
we need to create the fte, otherwise we need only to update it.

Signed-off-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>