staging:iio:ad7291 cleanup defines

Use BIT and GENMASK, remove unused defines, use AD7291_VOLTAGE_OFFSET
definition and move AD7291_BITS.

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>