ALSA: hda - Combine snd_hda_codec_flush_*_cache() to a single function

Since both snd_hda_codec_flush_amp_cache() and
snd_hda_codec_flush_cmd_cache() are called usually at the same time,
we can simply combine them to a single function,
snd_hda_codec_flush_cache().

Signed-off-by: Takashi Iwai <tiwai@suse.de>