drop_monitor: Consider all monitoring states before performing configuration

The drop monitor configuration (e.g., alert mode) is global, but user
will be able to enable monitoring of only software or hardware drops.

Therefore, ensure that monitoring of both software and hardware drops are
disabled before allowing drop monitor configuration to take place.

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