drm/nouveau/core: xtensa firmware size needs to be 0x40000 no matter what

The current logic is wrong since we send fw->size >> 8 to the
card. Rounding the size up by 0x100 and 0x1000 didn't seem to help,
the card still hung, so go back to what the blob does -- 0x40000.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>