ASoC: wm9713: use snd_soc_*() calls to update ac97 registers

Convert wm9713 to use the more modern registers manipulation functions,
such as snd_soc_read(), snd_soc_write() and snd_soc_update_bits().

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Reviewed-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>