igb: ethtool -d reads EICR which is incorrect as it is read on clear

Ethtool -d is reading the EICR and ICR registers which is currently
clearing these registers and masking off interrupts.  To prevent this we
read the EICS and ICS equivilents as they can be read without clearing or
masking.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>