ALSA: timer: Remove kernel warning at compat ioctl error paths

Some timer compat ioctls have NULL checks of timer instance with
snd_BUG_ON() that bring up WARN_ON() when the debug option is set.
Actually the condition can be met in the normal situation and it's
confusing and bad to spew kernel warnings with stack trace there.
Let's remove snd_BUG_ON() invocation and replace with the simple
checks.  Also, correct the error code to EBADFD to follow the native
ioctl error handling.

Reported-by: syzbot <syzkaller@googlegroups.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>