firewire: ohci: cache the context run bit

The DMA context run control bit is entirely controlled by software, so
it is safe to cache it.  This allows the driver to avoid doing an
additional MMIO read when queueing an AT packet.

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