[ALSA] make sound/isa/gus/gusextreme.c:devices static

'devices' is not a good name for a global variable.
Thankfully, it can become static.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>