ALSA: Force a cast to silence a warning from "sparse"

Some audio drivers are calling snd_dma_continuous_data(GFP_KERNEL)
which makes "sparse" give a warning:

  $ make C=2 M=sound/usb modules
    ...
  sound/usb/6fire/pcm.c:625:25: warning: cast from restricted gfp_t
  sound/usb/caiaq/audio.c:845:41: warning: cast from restricted gfp_t
  sound/usb/usx2y/usbusx2yaudio.c:997:54: warning: cast from restricted gfp_t
  sound/usb/usx2y/usbusx2yaudio.c:1001:54: warning: cast from restricted gfp_t
  sound/usb/usx2y/usx2yhwdeppcm.c:774:54: warning: cast from restricted gfp_t
  sound/usb/usx2y/usx2yhwdeppcm.c:778:54: warning: cast from restricted gfp_t

Add __force to the cast to silence the warning.

Signed-off-by: Antonio Ospite <ao2@amarulasolutions.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>