nouveau: fix function cast warning

Calling a function through an incompatible pointer type causes breaks
kcfi, so clang warns about the assignment:

drivers/gpu/drm/nouveau/nvkm/subdev/bios/shadowof.c:73:10: error: cast from 'void (*)(const void *)' to 'void (*)(void *)' converts to incompatible function type [-Werror,-Wcast-function-type-strict]
   73 |         .fini = (void(*)(void *))kfree,

Avoid this with a trivial wrapper.

Fixes: c39f472e ("drm/nouveau: remove symlinks, move core/ to nvkm/ (no code changes)")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240404160234.2923554-1-arnd@kernel.org