sound: Fix warnings relating to ignored return value in snd_card_register

Do not ignore the return of 'device_create_file' in
'snd_card_register' and thereby fixing the following warnings:

sound/core/init.c: In function 'snd_card_register':
sound/core/init.c:640: warning: ignoring return value of
'device_create_file', declared with attribute warn_unused_result
sound/core/init.c:641: warning: ignoring return value of
'device_create_file', declared with attribute warn_unused_result

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: Takashi Iwai <tiwai@suse.de>