ALSA: asihpi: Fix PCM format notations

asihpi driver treats -1 as an own invalid PCM format, but this needs
a proper cast with __force prefix since PCM format type is __bitwise.
Define a constant with the proper type and use it allover.

This fixes sparse warnings like:
  sound/pci/asihpi/asihpi.c:315:9: warning: incorrect type in initializer (different base types)

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