timestamp in RUNNING mode is already taken in update_hw_ptr routine,
getting a new timestamp introduces offset between hw_ptr, audio_tstamp
and system time

Add else condition to read timestamp as fallback and only when
enabled
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>