iio: adc: ad7091r: Set device mode through chip_info callback

iio: adc: ad7091r: Set device mode through chip_info callback

AD7091R-5 devices have a few modes of operation (sample, command,
autocycle) which are set by writing to configuration register fields.
Follow up patches will add support for AD7091R-2/-4/-8 which don't have
those operation modes nor the register fields for setting them.
Make ad7091r_set_mode() a callback function of AD7091R chip_info struct
so the base driver can appropriately handle each design without having
to check which actual chip is connected.

Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Link: https://lore.kernel.org/r/5140336980f66c2c45f05895c3b68e2f65fba1c2.1703013352.git.marcelo.schmitt1@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>