drm/nouveau/bios: return actual size of the buffer retrieved via _ROM

Fixes detection of a failed attempt at fetching the entire ROM image
in one-shot (a violation of the spec, that works a lot of the time).

Tested on a HP Zbook 15 G2.
Signed-off-by: NBen Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_acpi.c

@@ -367,6 +367,7 @@ static int nouveau_rom_call(acpi_handle rom_handle, uint8_t *bios,
 		return -ENODEV;
 	}
 	obj = (union acpi_object *)buffer.pointer;
+	len = min(len, (int)obj->buffer.length);
 	memcpy(bios+offset, obj->buffer.pointer, len);
 	kfree(buffer.pointer);
 	return len;