iwlwifi: add debugfs to control stuck queue timer

In current implementation, stuck queue timer is fixed to 1 second. Add
debugfs file to modify the timer to enhance the flexibility:

Set the monitor_period as following:
   0: disable stuck queue force reset function
   1 - 60000: monitor period (1 - 60 second)

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>