ALSA: emu10k1: set the "no filtering" bits on PCM voices on Audigy

ALSA: emu10k1: set the "no filtering" bits on PCM voices on Audigy

Given that the filter is already set to neutral for PCM voices, the
only observable effect is that the Z1/Z2/FXBUS registers don't have a
stray bit set for negative numbers anymore. The bit is below the ones
significant for output, but it would mess with 32-bit sample
recombination, which we intend to add.

kX-project does that, but I had to figure out myself why.

Signed-off-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Link: https://lore.kernel.org/r/20230715160802.326872-1-oswald.buddenhagen@gmx.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>