iio: adc: ina2xx: Make use of attribute flags consistent on all channels

Flags for shared channel attributes should be set on all channels of a
channel set. I.e. SAMP_FREQUENCY and OVERSAMPLING_RATIO are set on the
in_voltage{0,1} channels, thus should be set on in_power, in_current.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>