ssb: Fix all-ones boardflags

In the SSB SPROM a field set to all ones means the value
is not defined in the SPROM.
In case of the boardflags, we need to set them to zero
to avoid confusing drivers. Drivers will only check the
flags by ANDing.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Gabor Stefanik <netrolller.3d@gmail.com>
Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>