iwlwifi: seperate disconnected antenna function

Disconnected antenna algorithm is seperated into its own function from chain noise
calibration routine for better code management.

Signed-off-by: Shanyu Zhao <shanyu.zhao@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>