e1000: allow ethtool to pass arbitrary speed advertisment

With a patch, ethtool can now signify the driver to advertise more
than just a single speed/duplex setting. This allows you to tell the
card to advertise in 10/100 in any speed if you don't have a gigabit
switch for instance.

Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>