ixgbe: Fix Receive Address Register (RAR) cleaning and accounting

This fixes an issue when clearing out the RAR entries.  If RAR[0]
is the only address in use, don't clear the others.

Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>