iwlwifi: do not set tx power when channel is changing

Mac80211 can request for tx power and channel change in one ->config
call. If that happens, *_send_tx_power functions will try to setup tx
power for old channel, what can be not correct because we already change
the band. I.e  error  "Failed to get channel info for channel 140 [0]",
can be printed frequently when operating in software scanning mode.

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>