ASoC: img-spdif-out: Rename suspend / resume funcs

Rename suspend and resume functions to runtime_suspend and
runtime_resume, which is what they actually are. This will avoid
confusion when adding suspend and resume functions in a subsequent
patch.

Signed-off-by: Ed Blake <ed.blake@sondrel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>