iwlwifi: send tx_power_cmd synchronously

On 5xxx and 6xxx change to send tx_power_cmd command synchronously,
to do not start other commands when setting tx power is pending.
We currently do the same for 4956 and 3945.

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>