[PATCH] ALSA: au88x0: use explicitly signed char

[Al Viro]

On Mon, Oct 24, 2022 at 06:29:29PM +0200, Jason A. Donenfeld wrote:
> With char becoming unsigned by default, and with `char` alone being
> ambiguous and based on architecture, signed chars need to be marked
> explicitly as such. This fixes warnings like:

It might make sparse to STFU, but it does *not* resolve the underlying
issue:

vortex_adb_checkinout() returns a number in range of 0..31 on success
and -ENOMEM on failure.  Quite a few callers don't bother to check...