[ALSA] au88x0_synth.c bugfix

This patch fixes the code in vortex_wt_SetFrequency() to what seems to
have been intended.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>