mlxsw: Extract classification of router-related events to a helper

Several more events are coming in the following patches, and extending the
if statement is getting awkward. Instead, convert it to a switch.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>