bcma: implement BCM4331 workaround for external PA lines

We need to disable ext. PA lines for reading SPROM. It's disabled by
default, but this patch allows using bcma after loading wl, which leaves
workaround enabled.

Cc: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>