ASoC: rt5514: Guard Hotword Model bytes loading

If the rt5514-spi driver is not enabled in kernel, hotword model will
not be loaded when "DSP Voice Wake Up" is set to turn on DSP mode, and
an error is logged instead.

Signed-off-by: Hsin-Yu Chao <hychao@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>