ASoC: wm8993: Refactor set_pll code to avoid GCC warnings

Refactor set_pll code to avoid the following warnings:

sound/soc/codecs/wm8983.c:873:40: warning: 'pll_div.k' may be used uninitialized in this function [-Wuninitialized]
sound/soc/codecs/wm8983.c:870:9: warning: 'pll_div.n' may be used uninitialized in this function [-Wuninitialized]
sound/soc/codecs/wm8983.c:869:23: warning: 'pll_div.div2' may be used uninitialized in this function [-Wuninitialized]

Do the same as in commit 86ce6c9a (ASoC: WM8804: Refactor set_pll code to avoid
GCC warnings).

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>