igb: Do not overwrite mdicnfg register when accessing 82580 phy

This change removes the extra configuration we were doing on the mdicnfg
register which should be set by EEPROM and which we should not need to
write again afterwards.  This code was a holdover from some earlier
development work that was being done on a board with an incomplete EEPROM
and is no longer needed now that the production EEPROMs are in place.

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>