e1000e: Fix ethtool offline tests for 82579 parts

Changes to the rar_entry_count value require a change to the indexing
used to access the SHRA[H|L] registers when testing them with
'ethtool -t <iface> offline'

Signed-off-by: Dave Ertman <davidx.m.ertman@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>