firewire: cdev: add ioctl to query maximum transmission speed

While the speed of asynchronous transactions is automatically chosen by
the kernel, the speed of isochronous streams has to be chosen by the
initiating client.

In case of 1394a bus topologies, the maximum possible speed could be
figured out with some effort by evaluation of the remote node's link
speed field in the config ROM, the local node's link speed field, and
the PHY speeds and topologic information in the local node's or IRM's
topology map CSR.  However, this does not work in case of 1394b buses.

Hence add an ioctl to export the maximum speed which the kernel already
determined.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>