iio: dac: ad5758: Modifications for new revision

This patch will ensure compatibility with the new revision of the AD5758
dac converter. The modifications consist of removing the fault_prot_switch
function since this option is no longer available, and enabling the
ENABLE_PPC_BUFFERS bit in ADC_CONFIG register before setting the PPC
current mode.

The previous version of the chip was never released to customers
so there is no need to support it going forwards.

Signed-off-by: Mircea Caprioru <mircea.caprioru@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>