igb: cleanup igb loopback path

The code path for setting up phy loopback testing was out of date and was
setting bits it didn't need to.  This change cleans up the code path and
removes some code that has no effect on teh driver.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>