mlxsw: core: Move port module events enablement to a separate function

Use a separate function for enablement of port module events such
plug/unplug and temperature threshold crossing. The motivation is to
reuse the function for line cards.

Signed-off-by: Vadim Pasternak <vadimp@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>