wimax/i2400m: don't call netif_start_queue() in _tx_msg_sent()

Reported and fixed by Cindy H Kao.

When the device is stopped __i2400m_dev_stop() stops the network
queue.

However, when this is done in the middle of heavy network operation,
when the bus-specific subdriver is still wrapping up and it reports a
sent TX transaction with _tx_msg_sent() right after the device was
stopped, the queue was being started again, which was causing a stream
of oopsen and finally a panic.

In any case, said call has no place there. It's a left over from an
early implementation that was discarded later on.

Signed-off-by: Inaky Perez-Gonzalez <inaky@linux.intel.com>