nouveau: fix function cast warnings

nouveau: fix function cast warnings

clang-16 warns about casting between incompatible function types:

drivers/gpu/drm/nouveau/nvkm/subdev/bios/shadow.c:161:10: error: cast from 'void (*)(const struct firmware *)' to 'void (*)(void *)' converts to incompatible function type [-Werror,-Wcast-function-type-strict]
  161 |         .fini = (void(*)(void *))release_firmware,

This one was done to use the generic shadow_fw_release() function as a
callback for struct nvbios_source. Change it to use the same prototype
as the other five instances, with a trivial helper function that actually
calls release_firmware.

Fixes: 70c0f263cc2e ("drm/nouveau/bios: pull in basic vbios subdev, more to come later")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240213095753.455062-1-arnd@kernel.org