[ALSA] pcm_oss - fix SNDCTL_DSP_GETOPTR not working correctly

ALSA<-OSS emulation
This patch changes snd_pcm_oss_bytes() by adding a local variable
for the frames -> bytes conversion, which means that the frame count
is no longer corrupted by this conversion.

Signed-off-by: Roger Mach <bigmach@us.ibm.com>
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>