ALSA: emux: Add support of loading GUS-patch

It's a feature request for the ancient sutff, but it's still valid;
the loading of a GUS-patch isn't available via hwdep device although
it's supported over OSS sequencer.  The only missing piece is the call
of snd_soundfont_load_guspatch() in synth emux hwdep code.

Signed-off-by: Takashi Iwai <tiwai@suse.de>