radeon: Fix disabling PCI bus mastering on big endian hosts.

It would previously write basically random bits to PCI configuration space...
Not very surprising that the GPU tended to stop responding completely. The
resulting MCE even froze the whole machine sometimes.

Now resetting the GPU after a lockup has at least a fighting chance of
succeeding.

Cc: stable@vger.kernel.org
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>