iio: adc: xadc: Fix coding style violations

This patch fix following checkpatch warnings in xadc driver
1. prefer 'unsigned int' to bare use of 'unsigned'.
2. else is not generally useful after a break or return.
3. fill all function definition arguments with identifier name

Signed-off-by: Sai Krishna Potthuri <lakshmis@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>