igb: add flushes between RAR writes when setting mac address

There are some switches that will do write combining when they see two
sequential regions written. In order to avoid any possible write combining
issues it is necessary to add a flush after writing each piece of a rar
register.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>