iio:adc:berlin2-adc: use GENMASK and BIT for masks

Make use of GENMASK for consecutive bitmasks and BIT for single bitmasks.

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Acked-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>