iwl3945: fix wakeup interrupt

We have only 5 tx queues on 3945, updating il->txq[5] results in
writing random value to HBUS_TARG_WRPTR register.

Additionally use spin lock to protect txq->write_ptr and
txq->need_update fields also modified in TX path.

Tested-by: Pedro Francisco <pedrogfrancisco@gmail.com>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>