iio: adc: ad7768-1: set MOSI idle state to prevent accidental reset

Datasheet recommends Setting the MOSI idle state to high in order to
prevent accidental reset of the device when SCLK is free running.
This happens when the controller clocks out a 1 followed by 63 zeros
while the CS is held low.

Check if SPI controller supports SPI_MOSI_IDLE_HIGH flag and set it.

Fixes: a5f8c7da ("iio: adc: Add AD7768-1 ADC basic support")
Signed-off-by: Jonathan Santos <Jonathan.Santos@analog.com>
Reviewed-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Link: https://patch.msgid.link/c2a2b0f3d54829079763a5511359a1fa80516cfb.1741268122.git.Jonathan.Santos@analog.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>