iwlegacy: do not set tx power when channel is changing

Same fix as f844a709


"iwlwifi: do not set tx power when channel is changing".

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>