iio: adc: twl4030: Unexport twl4030_madc_conversion()

All madc users have been converted to IIO API, so drop the
legacy API. The function is still used inside of the driver.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Acked-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>