b43legacy: Fix error due to MMIO access with SSB unpowered

There is a dummy read of a PCI MMIO register that occurs before the SSB bus
has been powered, which is an error. This bug has not been seen earlier,
but was apparently exposed when udev was updated to version 182.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>