ASoC: Don't set invalid name string to snd_card->driver field

The snd_card->driver field contains a driver name string, and in
general it shouldn't contain space or special letters.  The commit
2b39535b9e54888649923beaab443af212b6c0fd changed the string copy from
card->name, but the long name string may contain such letters, thus
it may still lead to a segfault.

A temporary fix is not to copy the long name string but just keep it
empty as the earlier version did.

Reported-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>