mlxsw: spectrum: Initialize netdev's module overheat counter

The overheat counter is a per-module counter, but it is exposed as part
of the corresponding netdev's statistics. It should therefore be
presented to user space relative to the netdev's lifetime.

Query the counter just before registering the netdev, so that the value
exposed to user space will be relative to this initial value.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>