spi: fix device-node leaks

Make sure to release the device-node reference taken in
of_register_spi_device() on errors and when deregistering the device.

Fixes: 284b0189 ("spi: Add OF binding support for SPI busses")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>