mlxsw: spectrum_flower: Disable mixed bound blocks to contain action drop

Action drop is going to be tracked by two separate traps, one for
ingress and one for egress. Prepare for it and disallow the possibility
to have drop action in blocks which are bound to both ingress and
egress.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>