Since commit 2a9685d1 ("iio: adc: xilinx: use more devres helpers and
remove remove()") the `irq` field from XADC driver state struct is only
used in the `probe()` function.

Use the local `irq` variable throughout the `probe()` function and remove
the now unused field from the state struct.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20211017121551.24063-1-lars@metafoo.deSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>