drm/nouveau/bios: fetch the vbios from PROM using only aligned 32-bit accesses

drm/nouveau/bios: fetch the vbios from PROM using only aligned 32-bit accesses

Other kind of accesses are unreliable on Kepler cards. As advised by NVIDIA,
let's only use 32-bit accesses to fetch the vbios from PROM.

This fixes vbios fetching on my nve7 which failed in certain specific
conditions.

I suggest we Cc stable, for all kernels they still maintain after the big
rewrite.

Suggested-by: Christian Zander <[email protected]>
Signed-off-by: Martin Peres <[email protected]>
Signed-off-by: Ben Skeggs <[email protected]>