iio: adc: xilinx: Move request_irq before enabling interrupts

Enabling the Interrupts before registering the irq handler is a bad
idea. This patch corrects the same for XADC driver.

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