ASoC: wm9081: Remove #if IS_ENABLED(CONFIG_I2C)

wm9081 driver doesn't work without CONFIG_I2C anyway.
Let's remove #if IS_ENABLED(CONFIG_I2C)

And, this patch adds "depends on I2C" to Kconfig

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>