iwlwifi: 5150 compute ct kill threshold

This patch adds computation of ct kill threshold
for 5150. Threshold is computed from calibration
data in the EEPROM.

Signed-off-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>