Fix suggested by Benjamin Herrenschmidt <benh@kernel.crashing.org>

On architectures like PPC, char is handled as "unsigned char", thus the
pcm_format_data table entries with -1 give a positive 255.  This results
in Oops with OSS-emulation on such architectures.

The patch simply adds the right signed/unsigned prefix to fix this problem.
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>