spi: core: Fix error code in spi_register_controller()

Return -ENOMEM if the allocation fails.  Don't return success.

Fixes: 6598b91b ("spi: spi.c: Convert statistics to per-cpu u64_stats_t")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/Yqh6bdNYO2XNhPBa@kili


Signed-off-by: Mark Brown <broonie@kernel.org>