e1000e: correct MAC-PHY interconnect register offset for 82579

The MAC-PHY interconnect register set on ICH/PCH parts is accessed through
a peephole mechanism by writing an offset to a CSR register.  The offset
for the interconnect's half-duplex control register (which is used in a
jumbo frame workaround for 82579) is incorrect.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>