drivers/iio/dac/ad5270.c: Tristate SDO pin for SPI devices
When done configuring the device, place the SDO pin in high impedance to minimize the power dissipation and to avoid interference with SPI bus operations that apply to other devices.

Signed-off-by: Dan Nechita <dan.nechita@analog.com>
drivers/iio/adc/ad7791.c: Reset the part before configuring it
The reset returns the communication interface to a default state from which the part can be succesfully configured.

Signed-off-by: Dan Nechita <dan.nechita@analog.com>
drivers/iio/adc/ad_sigma_delta.c: The desired channel has to be selected in order to correctly fill the buffer with the corresponding data.