ALSA: sh: Fix compile warning wrt const

A long-standing compile warning was seen during build test:
  sound/sh/aica.c: In function 'load_aica_firmware':
  sound/sh/aica.c:521:25: warning: passing argument 2 of 'spu_memload' discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]

Fixes: 198de43d ("[ALSA] Add ALSA support for the SEGA Dreamcast PCM device")
Link: https://lore.kernel.org/r/20200105144823.29547-69-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/sh/aica.c

@@ -101,10 +101,10 @@ static void spu_memset(u32 toi, u32 what, int length)
 }
 
 /* spu_memload - write to SPU address space */
-static void spu_memload(u32 toi, void *from, int length)
+static void spu_memload(u32 toi, const void *from, int length)
 {
 	unsigned long flags;
-	u32 *froml = from;
+	const u32 *froml = from;
 	u32 __iomem *to = (u32 __iomem *) (SPU_MEMORY_BASE + toi);
 	int i;
 	u32 val;