ALSA: pcm - Fix regressions with VMware

VMware tends to report PCM positions and period updates at utterly
wrong timing.  This screws up the recent PCM core code that tries
to correct the position based on the irq timing.

Now, when a backward irq position is detected, skip the update
instead of rebasing.  (This is almost the old behavior before
2.6.30.)

Signed-off-by: Takashi Iwai <tiwai@suse.de>