snd_mixer_oss_build_input(): fix for __you_cannot_kmalloc_that_much failure with gcc-3.2

Rework this functions so that gcc-3.2 can successfully perform
constant-folding.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Jaroslav Kysela <perex@suse.cz>
Cc: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>