iio: adc: twl4030: Fold twl4030-madc.h into driver

twl4030-madc.h is no longer used by anything outside of
the iio driver, so it can be merged into 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>