[ALSA] Fix PCM MMAP time-stamp mode

When MMAP time-stamp mode is given, it's supposed to update the time-stamp
only at period boundary.  However, it currently updates at each status call
so this is just useless.  The patch fixes this misbehavior.
Also it fixes the wrong check of tstamp_mode (don't use bit-and for enum).

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