igb: Some fine tuning

This patch does the following:

1. Changes the existing supported device id's so now DH89xxCC is not supported when EEPROM is not read.
2. Adds two more device ids for DH89xxCC in backplane mode and SFP.
3. Driver now initializes previously possibly uninitialized value in igb_reset_mdicnfg_82580().

Signed-off-by: Joseph Gasparakis <joseph.gasparakis@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>