Fix module parameter data type to eliminate build warnings.

sound/isa/opti9xx/opti92x-ad1848.c:87:1: warning: return from incompatible pointer type
sound/isa/opti9xx/opti92x-ad1848.c:87:1: warning: return from incompatible pointer type
Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Takashi Iwai <tiwai@suse.de>