ALSA: emux: fix /proc teardown at module unload

We forgot to remember the wavetable /proc entry, so we'd fail to free it
at module unload.

This matters only when only the synth module is unloaded, as unloading
the card driver would tear down the sub-entry anyway.

Signed-off-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Message-ID: <20240406064830.1029573-2-oswald.buddenhagen@gmx.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>