iwlwifi: remove extra error msg on sensitivity calibration

Do not need to log error when fail the sensitivity command, driver will send
sensitivity write command to uCode after each sensitivity calibration if
station is associated with AP. It is a normal case when user unload the module
or shutdown the system while still associated with the AP, since uCode already
on the way down, it will not reply the sensitivity write request; report
error in this case will give misleading information, remove the error checking
here to provide a clean shutdown if no other error detected.

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>