ALSA: sh: Fix compile warning wrt const

ALSA: sh: Fix compile warning wrt const

[ Upstream commit f1dd4795b1523fbca7ab4344dd5a8bb439cc770d ]

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: 198de43d758c ("[ALSA] Add ALSA support for the SEGA Dreamcast PCM device")
Link: https://lore.kernel.org/r/20200105144823.29547-69-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>