mlxsw: reg: Add new trap actions

Subsequent patches will add discard traps support in mlxsw. The driver
cannot configure such traps with a normal trap action, but needs to use
exception trap action, which also increments an error counter.

On the other hand, when these traps are initialized or set to drop
action, they should use the default drop action set by the firmware.
This guarantees that when the feature is disabled we get the exact same
behavior as before the feature was introduced.

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