[ALSA] ASoC - Fix build warnings in soc-core.c

This patch fixes some build warnings in soc-core.c
Changes:-
 o Check the return value of soc_ac97_dev_register()
 o Check return value of calls to device_create_file()

Signed-off-by: Liam Girdwood <liam.girdwood@wolfsonmicro.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>