igb: Code to prevent overwriting SFP I2C

This patch fixes "overwrite" problem. without this fix, SFP I2C EEPROM
data, which is located at A0 can be overwritten by the phy write function.

Signed-off-by: "Akeem G. Abodunrin" <akeem.g.abodunrin@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>