e1000: fix loopback ethtool test

Ethtool was reporting that loopback failed randomly on esb2
systems. Upon study it was found that the phy manual was changed
with respect to the loopback mode bits. The new value fixes it.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>