ALSA: emu10k1: simplify tone control switch DSP code

Instead of using lots of instructions to mix wet and dry signals,
simply skip over the whole code block if tone control is disabled.
This also allows us doing away with the "shadow" playback channels.
Tested-by: Jonathan Dowland <jon@dow.land>
Signed-off-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Link: https://lore.kernel.org/r/20230510173917.3073107-7-oswald.buddenhagen@gmx.deSigned-off-by: Takashi Iwai <tiwai@suse.de>