staging: iio: ad9834: add a check for devm_clk_get

ad9834_probe misses a check for devm_clk_get and may cause problems.
Add a check like what ad9832 does to fix it.

Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>