ALSA: pcm: Remove VLA usage

A helper function used by snd_pcm_hw_refine() still keeps using VLA
for timestamps of hw constraint rules that are non-fixed size.

Let's replace the VLA with a simple kmalloc() array.

Reference: https://lkml.org/lkml/2018/3/7/621


Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>