ssb: use u16 for storing board rev

Specs say about size 2 (u16) and my 14e4:4727 has board rev 0x1211.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>