ALSA: cs46xx: mark snd_cs46xx_download_image as static

ALSA: cs46xx: mark snd_cs46xx_download_image as static

snd_cs46xx_download_image() was originally called from dsp_spos.c, but
is now local to cs46xx_lib.c. Mark it as 'static' to avoid a warning
about it lacking a declaration, and '__maybe_unused' to avoid a warning
about it being unused when CONFIG_SND_CS46XX_NEW_DSP is disabled:

sound/pci/cs46xx/cs46xx_lib.c:534:5: error: no previous prototype for 'snd_cs46xx_download_image'

Fixes: 89f157d9e6bf ("[ALSA] cs46xx - Fix PM resume")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20230516195046.550584-1-arnd@kernel.org
Signed-off-by: Takashi Iwai <tiwai@suse.de>