iwlwifi: defer update power mode while scan

Do not set power mode when scanning, and defer that when scan finish.
We still set power mode in force case i.e. when device is overheated.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>