forcedeth: fix kexec regression

Fix regression tracked as http://bugzilla.kernel.org/show_bug.cgi?id=11361
and caused by commit f735a2a1

 ("[netdrvr]
forcedeth: setup wake-on-lan before shutting down") that makes network
adapters integrated into the NVidia MCP55 chipsets fail to work in kexeced
kernels.  The problem appears to be that if the adapter is put into D3_hot
during ->shutdown(), it cannot be brought back into D0 after kexec (ref.
http://marc.info/?l=linux-kernel&m=121900062814967&w=4).  Therefore, only
put forcedeth into D3 during ->shutdown() if the system is to be powered
off.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Tested-by: Yinghai Lu <yhlu.kernel@gmail.com>
Cc: Ayaz Abdulla <aabdulla@nvidia.com>
Acked-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>