[ALSA] ad1816a: Fix modprobe snd_mpu401 && modprobe snd_ad1816a

   The ad1816a driver fails if the mpu401 driver has been loaded first. This
patch against linux 2.6.20 fixes it by just ignoring the MPU-401 device in
that case, so that the rest of the sound card can be used. The ad1816a
driver already handles the MPU-401 device being unavailable due to lack of
resources in the same way.

Signed-off-by: Rask Ingemann Lambertsen <rask@sygehus.dk>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>