[ALSA] sound/isa/ad1816a/ad1816a.c: check kmalloc() return value

Check the return value of kmalloc() in function snd_card_ad1816a_pnp(),
in file sound/isa/ad1816a/ad1816a.c.

Signed-off-by: Amit Choudhary <amit2030@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>