iwlwifi: change the default behavior of watchdog timer

The current default watchdog timer is enabled, but we are seeing issues on
legacy devices. So change the default setting of watchdog timer to per
device based. But user still can use the "wd_disable" module parameter
to overwrite the system setting

Cc: stable@vger.kernel.org #3.0+
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>