iwlwifi: use meaningful vars in _iwl_poll_bit()

Rename vars in _iwl_poll_bit() to better reflect the truth.

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>