Fix Tx queue flushing in iwm(4).

Fix Tx queue flushing in iwm(4).

iwm(4) was still using an outdated version of the TX_FLUSH command.
Current firmware expects a different version which has the same size but
different semantics. The iwx(4) driver is already using the new version.

Also do not log errors if flushing Tx queues fails. This can happen if
the AP disappeared. Just cope by resetting the device and clearing rings.

Should fix "flushing Tx queues failed" and related firmware errors.