iwlwifi: remove unused max_nrg_cck from sensitivity and constify

The sensitivity parameters are never modified, so they
should be const. Also remove the unused max_nrg_cck
value to save some space.

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