ALSA: fix excessive background noise introduced by OSS emulation rate shrink

Incorrect variable was used to get the next sample which caused S2
to be stuck with the same value resulting in loud background noise.

Signed-off-by: Steve Chen <schen at mvista.com>
Cc: <stable@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>