iio: adc: xilinx: check return value of xadc_write_adc_reg

In function xadc_probe, xadc_write_adc_reg can return an error value
when write fails. The fix checks for the return value consistent with
other invocations of the latter function.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Reviewed-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>