iwlwifi: do not use shadow registers by default

Shadow registers in the device are meant to
allow the driver to update certain device
registers without needing to wake up all
components of the device. However, using
this feature in the device causes
communication between the driver and the
device to become unreliable, resulting in
host command timeouts.

Disable this feature by default till a fix is
available for the bug.

Cc: stable@vger.kernel.org #2.6.38+
Signed-off-by: Meenakshi Venkataraman <meenakshi.venkataraman@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>