ALSA: SOC: Fix setting codec register with debugfs filesystem merge error

Call device_create_file only once in snd_soc_dapm_sys_add function.

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>