firewire: ohci: replace vm_map_ram() with vmap()

vm_map_ram() is intended for short-lived objects, so using it for the AR
buffers could fragment address space, especially on a 32-bit machine.
For an allocation that lives as long as the device, vmap() is the better
choice.

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