iio: xilinx-xadc: Fix clearing interrupt when enabling trigger

When enabling the trigger and unmasking the end-of-sequence (EOS) interrupt
the EOS interrupt should be cleared from the status register. Otherwise it
is possible that it was still set from a previous capture. If that is the
case the interrupt would fire immediately even though no conversion has
been done yet and stale data is being read from the device.

The old code only clears the interrupt if the interrupt was previously
unmasked. Which does not make much sense since the interrupt is always
masked at this point and in addition masking the interrupt does not clear
the interrupt from the status register. So the clearing needs to be done
unconditionally.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Fixes: bdc8cda1 ("iio:adc: Add Xilinx XADC driver")
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>