ASoC: rockchip: Fix an uninitialized variable compile warning

Paper over a trivial case leading to an uninitialized variable compile
warning:
  sound/soc/rockchip/rockchip_pdm.c:179:3: warning: ‘clk_out’ may be used uninitialized in this function [-Wmaybe-uninitialized]

Fixes: 624e8e00 ("ASoC: rockchip: pdm: fixup pdm fractional div")
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Mark Brown <broonie@kernel.org>