ALSA: aica: Fix a long-time build breakage

The build of aica sound driver has been broken since the timer API
conversion and some code rewrite.  This patch fixes the breakage by
using the common substream field, as well as a bit cleaning up wrt the
timer handling in the code.

Fixes: d522bb6a

 ("ALSA: sh: aica: Convert timers to use timer_setup()")
Signed-off-by: Takashi Iwai <tiwai@suse.de>