ALSA: pcm: Unify delay calculation in snd_pcm_status() and snd_pcm_delay()

Yet another slight code cleanup: there are two places where
calculating the PCM delay, and they can be unified in a single
helper.  It reduces the multiple open codes.

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