ASoC: wm8985: add i2c dependency

The wm8985 driver is now user-selectable, but building it with
I2C disabled results in a link failure:

sound/built-in.o: In function `wm8985_i2c_probe':
:(.text+0x44914): undefined reference to `__devm_regmap_init_i2c'
sound/built-in.o: In function `wm8985_exit':
:(.exit.text+0x3d8): undefined reference to `i2c_del_driver'
sound/built-in.o: In function `wm8985_modinit':
:(.init.text+0x1454): undefined reference to `i2c_register_driver'

This adds a Kconfig dependency the way that the other codec
drivers have it.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 811e66de

 ("ASoC: wm8985: add support for WM8758")
Signed-off-by: Mark Brown <broonie@kernel.org>