iio: adc: twl4030: Fix ADC[3:6] readings

MADC[3:6] reads incorrect values without these two following changes:

- enable the 3v1 bias regulator for ADC[3:6]
- configure ADC[3:6] lines as input, not as USB

Signed-off-by: Adam YH Lee <adam.yh.lee@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>