[ALSA] sound: snd_register_device_for_dev fix

snd_register_device_for_dev() can oops when device_create() returns
ERR_PTR(err).
Scenario:
preg->dev = device_create(...); /* fails */
if (preg->dev) /* contains ERR_PTR(err) */
	dev_set_drvdata(preg->dev, private_data);
and dev_set_drvdata() looks like this:
static inline void
dev_set_drvdata (struct device *dev, void *data)
{
	dev->driver_data = data; <--- boom
}
This patch should prevent that.

Signed-off-by: Mariusz Kozlowski <m.kozlowski@tuxland.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>