ASoC: wm8904: fix automatic sysclk configuration

The simple-card tries to signal the codec to disable rate constraints,
see commit 2458adb8 ("SoC: simple-card-utils: set 0Hz to sysclk when
shutdown"). This wasn't handled by the codec, instead it would set the
FLL frequency to 0Hz which isn't working. Since we don't have any rate
constraints just ignore this request.

Fixes: 13409d27

 ("ASoC: wm8904: configure sysclk/FLL automatically")
Signed-off-by: Michael Walle <michael@walle.cc>
Acked-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20191122232532.22258-1-michael@walle.cc
Signed-off-by: Mark Brown <broonie@kernel.org>