e1000e: Fix SHRA register access for 82579

Previous commit c3a0dce3 fixed an overrun for the RAR on i218 devices.
This commit also attempted to homogenize the RAR/SHRA access for all parts
accessed by the e1000e driver.  This change introduced an error for
assigning MAC addresses to guest OS's for 82579 devices.

Only RAR[0] is accessible to the driver for 82579 parts, and additional
addresses must be placed into the SHRA[L|H] registers.  The rar_entry_count
was changed in the previous commit to an inaccurate value that accounted
for all RAR and SHRA registers, not just the ones usable by the driver.

This patch fixes the count to the correct value and adjusts the
e1000_rar_set_pch2lan() function to user the correct index.

Cc: John Greene <jogreene@redhat.com>
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>