b43: Workaround invalid bluetooth settings

This adds a workaround for invalid bluetooth SPROM settings
on ASUS PCI cards.
This will stop the microcode from poking with the BT GPIO line.
This fixes data transmission on this device, as the BT GPIO line
is used for something TX related on this device
(probably the power amplifier or the radio).
This also adds a modparam knob to help debugging this in the future,
as more devices with this bug may show up.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>