[NIU]: More BMAC alt MAC address fixes.

From: Matheos Worku <Matheos.Worku@Sun.COM>

1) niu_enable_alt_mac() needs to be adjusted so that the mask
   is computed properly for the BMAC case.

2) BMAC has 6 alt MAC addresses available, not 7.

Signed-off-by: David S. Miller <davem@davemloft.net>