ALSA: hdspm - Fix compile warnings with PPC

The char can be unsigned on some architectures.  Since the code checks
the negative values, they should be declared as signed char explicitly.

  sound/pci/rme9652/hdspm.c:5449: warning: comparison is always false due to limited range of data type
  sound/pci/rme9652/hdspm.c:5462: warning: comparison is always false due to limited range of data type

Signed-off-by: Takashi Iwai <tiwai@suse.de>