iio: adc: twl4030_madc: Fix calculation of the temperature sense current

The bit mask to read the setting of the constant current source
for measuring the NTC voltage was the wrong one. Since default
value is initialized to the lowest level (000 = 10uA) the difference
was probably never noticed in practice.

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Marek Belisko <marek@goldelico.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>