e1000: test link state conclusively

e1000 was using one particular way to detect link, but with the advent
of some of the newer hardware designs using SERDES connections, tests
for link must completely cover all cases.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>