ALSA: sparc: Add missing kfree in error path

If 'of_ioremap' fails, then 'amd' should be freed, otherwise, there is a
memory leak.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Takashi Iwai <tiwai@suse.de>