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

sound/isa/sb/emu8000_pcm.c

@@ -435,7 +435,7 @@ enum {
 #define LOOP_WRITE(rec, offset, _buf, count, mode)		\
 	do {							\
 		struct snd_emu8000 *emu = (rec)->emu;		\
-		unsigned short *buf = (unsigned short *)(_buf); \
+		unsigned short *buf = (__force unsigned short *)(_buf); \
 		snd_emu8000_write_wait(emu, 1);			\
 		EMU8000_SMALW_WRITE(emu, offset);		\
 		while (count > 0) {				\
@@ -492,7 +492,7 @@ static int emu8k_pcm_silence(struct snd_pcm_substream *subs,
 #define LOOP_WRITE(rec, pos, _buf, count, mode)				\
 	do {								\
 		struct snd_emu8000 *emu = rec->emu;			\
-		unsigned short *buf = (unsigned short *)(_buf);		\
+		unsigned short *buf = (__force unsigned short *)(_buf);	\
 		snd_emu8000_write_wait(emu, 1);				\
 		EMU8000_SMALW_WRITE(emu, pos + rec->loop_start[0]);	\
 		if (rec->voices > 1)					\