mlxsw: core: Convert a series of if statements to switch case

Convert a series of if statements that handle different events to a
switch case statement. Encapsulate the per-event code in different
functions to simplify the code.

This is a preparation for subsequent patches that will add more events
that need to be handled.

Signed-off-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>