ALSA: ad1816a: Fix sparse warning wrt PCM format type

The PCM format type is with __bitwise, and it can't be converted from
integer implicitly.  Instead of an ugly cast, declare the function
argument of snd_ad1816a_get_format() with the proper snd_pcm_format_t
type.

This fixes the sparse warning like:
  sound/isa/ad1816a/ad1816a_lib.c:93:14: warning: restricted snd_pcm_format_t degrades to integer

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