net/mlx5: DR, Fix check for unsupported fields in match param

The existing loop doesn't cast the buffer while scanning it, which
results in out-of-bounds read and failure to create the matcher.

Fixes: 941f1979

 ("net/mlx5: DR, Add check for unsupported fields in match param")
Signed-off-by: Yevgeny Kliteynik <kliteyn@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>