net/mlx5: DR, Fix matcher builders select check

When selecting a matcher ste_builder_arr will always be evaluated
as true, instead check if num_of_builders is set for validity.

Fixes: 667f2646

 ("net/mlx5: DR, Support IPv4 and IPv6 mixed matcher")
Signed-off-by: Alex Vesker <valex@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>