pcm: hw: add a helper function to request hwsync without side-effects

pcm: hw: add a helper function to request hwsync without side-effects

SNDRV_PCM_IOCTL_SYNC_PTR command for ioctl(2) with
SNDRV_PCM_SYNC_PTR_HWSYNC flag has an effect to update hw_ptr.
This is an alternative of SNDRV_PCM_IOCTL_HWSYNC but caller can get
current status simultaneously.

This commit adds a helper function just to issue hwsync. To avoid
side-effect to change appl_ptr and avail_min, this commit uses
SNDRV_PCM_SYNC_PTR_APPL and SNDRV_PCM_SYNC_PTR_AVAIL_MIN flags.

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