firewire-ohci: work around oversized DMA reads on JMicron controllers

At least some JMicron controllers issue buggy oversized DMA reads when
fetching context descriptors, always fetching 0x20 bytes at once for
descriptors which are only 0x10 bytes long. This is often harmless, but
can cause page faults on modern systems with IOMMUs:

DMAR: [DMA Read] Request device [05:00.0] fault addr fff56000 [fault reason 06] PTE Read access is not set
firewire_ohci 0000:05:00.0: DMA context IT0 has stopped, error code: evt_descriptor_read

This works around the problem by always leaving 0x10 padding bytes at
the end of descriptor buffer pages, which should be harmless to do
unconditionally for controllers in case others have the same behavior.

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