igb: only read phy specific stats if in internal phy mode

There are a couple statistics registers that are not meant to
be read when in SGMII/serdes mode. This patch adds a check to
verify mode before reading and updating these statistics.

Signed-off-by: Nicholas Nunley <nicholasx.d.nunley@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>