iio: adc: xilinx: Remove dead code from xadc_zynq_setup

This patch removes dead code from xadc_zynq_setup. The condition
"if (tck_rate > XADC_ZYNQ_TCK_RATE_MAX)" cannot be true at any point of
time. There is also an incompatible parameter used in the code.
This patch fixes the same reported by coverity.

Signed-off-by: Manish Narani <manish.narani@xilinx.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>