ssb: Clean up extraction of MAC addresses from SPROM

ssb: Clean up extraction of MAC addresses from SPROM

Only rev 1 and 2 ssb SPROMs have fields named et0mac and et1mac;
however, all of the extraction routines extract pseudo data for these
fields from regions that are all 1's resulting in a hardware address
of FF:FF:FF:FF:FF:FF. This patch forces such a fill at the beginning of
the data extraction process, and only does the formal extraction if the
SPROM rev is 1 or 2.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>