iio: adc: ad7091r: Set alert bit in config register

iio: adc: ad7091r: Set alert bit in config register

The ad7091r-base driver sets up an interrupt handler for firing events
when inputs are either above or below a certain threshold.
However, for the interrupt signal to come from the device it must be
configured to enable the ALERT/BUSY/GPO pin to be used as ALERT, which
was not being done until now.
Enable interrupt signals on the ALERT/BUSY/GPO pin by setting the proper
bit in the configuration register.

Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Link: https://lore.kernel.org/r/e8da2ee98d6df88318b14baf3dc9630e20218418.1702746240.git.marcelo.schmitt1@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>