ALSA: aw2: avoid casting function pointers

ALSA: aw2: avoid casting function pointers

clang-16 started warning about incompatible function pointers here:

sound/pci/aw2/aw2-alsa.c:363:11: error: cast from 'void (*)(struct snd_pcm_substream *)' to 'snd_aw2_saa7146_it_cb' (aka 'void (*)(void *)') converts to incompatible function type [-Werror,-Wcast-function-type-strict]
  363 |                                                     (snd_aw2_saa7146_it_cb)
      |                                                     ^~~~~~~~~~~~~~~~~~~~~~~
  364 |                                                     snd_pcm_period_elapsed,
      |                                                     ~~~~~~~~~~~~~~~~~~~~~~
sound/pci/aw2/aw2-alsa.c:392:10: error: cast from 'void (*)(struct snd_pcm_substream *)' to 'snd_aw2_saa7146_it_cb' (aka 'void (*)(void *)') converts to incompatible function type [-Werror,-Wcast-function-type-strict]
  392 |                                                    (snd_aw2_saa7146_it_cb)
      |                                                    ^~~~~~~~~~~~~~~~~~~~~~~
  393 |                                                    snd_pcm_period_elapsed,
      |                                                    ~~~~~~~~~~~~~~~~~~~~~~

Add a forward declaration for struct snd_pcm_substrea to allow it to just
use the correct prototype.

Fixes: 98f2a97f207a ("[ALSA] Emagic Audiowerk 2 ALSA driver.")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20240213101327.460191-1-arnd@kernel.org
Signed-off-by: Takashi Iwai <tiwai@suse.de>