iio: adc: xilinx: use devm_krealloc() instead of kfree() + kcalloc()

We now have devm_krealloc() in the kernel Use it indstead of calling
kfree() and kcalloc() separately.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Tested-by: Anand Ashok Dumbre <anandash@xilinx.com>
Reviewed-by: Anand Ashok Dumbre <anandash@xilinx.com>
Link: https://lore.kernel.org/r/20201130142759.28216-3-brgl@bgdev.pl
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>