iio: adc: Fix bad GENMASK use, typos, whitespace

Commit 7906dd52 ("iio: ina2xx: Fix whitespace and re-order code")
changed the register number of the MASK_ENABLE register from 0x06 to the
value equivalent GENMASK(2,1), although its no mask.
Also fix a typo (INA2_6_6 instead of INA2_2_6), and use the datasheet
name ("Mask/Enable") for the register number define.
Fix bad indentation for channel attributes.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Acked-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>