NET/e1000: Fix powering off during shutdown

Prevent e1000 from putting the adapter into D3 during shutdown except when
we're going to power off the system, since doing that may generally cause
problems with kexec to happen (such problems were observed for igb and
forcedeth).  For this purpose seperate e1000_shutdown() from e1000_suspend()
and use the appropriate PCI PM callbacks in both of them.

Signed-off-by: "Rafael J. Wysocki" <rjw@sisk.pl>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>