ALSA: emu8000: Fix the cast to __user pointer

Fixes the sparse warnings.  The cast to __user pointer needs __force:
  sound/isa/sb/emu8000_pcm.c:528:9: warning: cast removes address space '<asn:1>' of expression

No functional changes, just sparse warning fixes.

Link: https://lore.kernel.org/r/20200206163152.6073-4-tiwai@suse.de


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