ssb: Silence warning for unknown backplane revision

When using a BCM4318 in a PCMCIA format, I get a startup message that the
device uses backplane revision 0xF000000. Next a WARNING is logged. Despite
the message, the device works fine, This patch silences the warning.

Cc: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>