e1000e: Fix ethtool register test code

A merge/cleanup code accidentally dropped 8254x code in and removed
8257x code here. Undo this mistake and use the pci-e relevant register
test similar as to what is in e1000.

Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>