bcma: extract antenna gains from SPROM correctly

Just like in case of SSB SPROMs they are encoded in a bit tricky way.
SPROM struct already uses s8 type and it's supposed to store decoded
values.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>