net/mlx4: fix UDP flow rule limitation enforcement

net/mlx4: fix UDP flow rule limitation enforcement

For some unknown reason, priorities do not have any effect on flow rules
that happen to match UDP destination ports. Those are always matched first
regardless.

This patch is a workaround that enforces this limitation at the PMD level;
such flow rules can only be created at the highest priority level for
correctness.

Fixes: a5171594fc3b ("net/mlx4: expose support for flow rule priorities")
Cc: stable@dpdk.org
Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>