drm/nouveau/devinit: Convert function disable() to be void

The current design of callback function disable() of struct
nvkm_devinit_func is defined to return a u64 value. In its implementation
in the driver modules, the function always returns a fixed value 0. Hence
the design and implementation of this function should be enhanced to return
void instead of a fixed value. This change also eliminates untouched
return variables.

The change is identified using the returnvar.cocci Coccinelle semantic
patch script.

Signed-off-by: Deepak R Varma <drv@mailo.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/Y9FFoooIXjlr+UP1@ubun2204.myguest.virtualbox.org