[ALSA] kill dead code

ALSA<-OSS emulation
The Coverity checker found this obviously dead code.

I'm not sure which of the if (plugin == NULL) is correct - this patch
removes the one that couldn't be true.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>