iwlwifi: fix time interval misuse in iwl_poll_{direct_}bit

The patch fixes the misuse of microsecond with millisecond in the
polling mechanism of the iwlwifi driver. The impact of this problem
is the unacceptable latency for the whole system (especially during
bringing down the wlan interface).

Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Acked-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>