bcma: handle alternative SPROM location

Some cards do not use additional 0x30 offset for SPROM location. We do
not know the real condition for it yet, make it BCM4331 specific for
now.

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