iwlwifi: do not clear data after chain noise calib

Chain noise calibration data are cleared after the calibration is done
in iwlagn_gain_computation() and iwl4965_gain_computation(). This cause
the debugfs entries for those data useless. To provide valid debugging
info, clear those data right before starting the calibration instead.

Signed-off-by: Shanyu Zhao <shanyu.zhao@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>