net/mlx5: unify validation of drop action

net/mlx5: unify validation of drop action

According to PRM: "Drop action is mutually-exclusive with any other
action, except for Count action".
In current code this limitation is checked separately in validation
function of each action.

This patch removes the discrete checks, and adds a single check common
for all actions.

Signed-off-by: Dekel Peled <dekelp@mellanox.com>
Acked-by: Ori Kam <orika@mellanox.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>