gusextreme depends on opl3 support. Add the approriate select to Kconfig.
Also remove the unnecessary hwdep select.

Relevant build errors:
ERROR: "snd_opl3_hwdep_new" [sound/isa/gus/snd-gusextreme.ko] undefined!
ERROR: "snd_opl3_create" [sound/isa/gus/snd-gusextreme.ko] undefined!
Signed-off-by: NVille Syrjala <syrjala@sci.fi>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>