ohci1394: make phys_dma parameter read-only

Being able to switch physical DMA on and off at run time would be a nice
feature but a PITA to support by highlevel drivers and userspace apps.
Therefore allow it only to be set when the driver is being loaded.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Ben Collins <bcollins@ubuntu.com>