iwlwifi: add debugfs to disable/enable run time calibration

This patch adds functionality to debugfs to enable or disable chain
noise or sensitivity calibrations.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>