spi: core: Do not mangle error code from kthread_run()

kthread_run() could return ERR_PTR(-EINTR) from kthread_create_on_node().
Return the actual error code in spi_init_queue() instead of mangling it to
-ENOMEM.

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>