net/mlx5: Refactor find_flow_rule

The way we compare between two dests will need to be used in other
places in the future, so we factor out the comparison logic
between two dests into a separate function.

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