ALSA: PCM midlevel: Do not update hw_ptr_jiffies when hw_ptr is not changed

Some hardware might have bigger FIFOs and DMA pointer value will be updated
in large chunks. Do not update hw_ptr_jiffies and position timestamp when
hw_ptr value was not changed.

Signed-off-by: Jaroslav Kysela <perex@perex.cz>
Signed-off-by: Takashi Iwai <tiwai@suse.de>