e1000e: fix and commonize code for setting the receive address registers

Fix e1000e_rar_set() to flush consecutive register writes to avoid write
combining which some parts cannot handle.  Update e1000e_init_rx_addrs()
to call the fixed e1000e_rar_set() instead of duplicating code.

Also change e1000e_rar_set() to _not_ set the Address Valid bit if the MAC
address is all zeros.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>