media/bt8xx: Remove old powerpc cruft

The powerpc arch code enables PCI_COMMAND_MEMORY (and has done so for
more than 10 years at least !) on pci_enable_device() and the hackery
on the MMIO accessor is useless as well, our writel does everything
this driver should need.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>