iio: ad7793: Fix the serial interface reset

The serial interface can be reset by writing 32 consecutive 1s to the device.
'ret' was initialized correctly but its value was overwritten when
ad7793_check_platform_data() was called. Since a dedicated reset function
is present now, it should be used instead.

Fixes: 2edb769d ("iio:ad7793: Add support for the ad7798 and ad7799")
Signed-off-by: Dragos Bogdan <dragos.bogdan@analog.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>