e1000e: Make speed detection on hotplugging cable more reliable

After hot plugging an 1Gbps Ethernet cable with 1Gbps link partner, the
MII_BMSR may report 10Mbps, renders the network rather slow.

The issue has much lower fail rate after commit 59653e64 ("e1000e:
Make watchdog use delayed work"), which essentially introduces some
delay before running the watchdog task.

But there's still a chance that the hot plugging event and the queued
watchdog task gets run at the same time, then the original issue can be
observed once again.

So let's use mod_delayed_work() to add a deterministic 1 second delay
before running watchdog task, after an interrupt.

Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>