clk: wm831x: Provide is_prepared() rather than is_enabled()

Since the driver was written an is_prepared() operation has been made
possible. Since the driver uses I2C I/O only prepare operations are
provided so move the is_enabled() operation over to is_prepared().

Signed-off-by: Mark Brown <broonie@linaro.org>
Signed-off-by: Mike Turquette <mturquette@linaro.org>