igb: add locking to reads of the i2c interface

The current implementation of sgmii support isn't correctly locking the
interfaces for reads/writes.  This change pulls the read/write
functionality out of 82575.c and moves it to phy.c.  In addition it
replaces the implementation in 82575.c with one that uses locking around
the relocated i2c interface calls.

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>