igb: use correct bits to identify if managability is enabled

igb was previously checking the wrong bits in the MANC register to determine
if managability was enabled.  As a result it was incorrectly powering down and
resetting the phy when it didn't need to.

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>