iio: imx8qxp-adc: fix irq flood when call imx8qxp_adc_read_raw()

iio: imx8qxp-adc: fix irq flood when call imx8qxp_adc_read_raw()

irq flood happen when run
    cat /sys/bus/iio/devices/iio:device0/in_voltage1_raw

imx8qxp_adc_read_raw()
{
	...
	enable irq
	/* adc start */
	writel(1, adc->regs + IMX8QXP_ADR_ADC_SWTRIG);
	^^^^ trigger irq flood.
	wait_for_completion_interruptible_timeout();
	readl(adc->regs + IMX8QXP_ADR_ADC_RESFIFO);
	^^^^ clear irq here.
	...
}

There is only FIFO watermark interrupt at this ADC controller.
IRQ line will be assert until software read data from FIFO.
So IRQ flood happen during wait_for_completion_interruptible_timeout().

Move FIFO read into irq handle to avoid irq flood.

Fixes: 1e23dcaa1a9f ("iio: imx8qxp-adc: Add driver support for NXP IMX8QXP ADC")
Cc: stable@vger.kernel.org
Signed-off-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Cai Huoqing <cai.huoqing@linux.dev>
Reviewed-by: Haibo Chen <haibo.chen@nxp.com>
Link: https://lore.kernel.org/r/20221201140110.2653501-1-Frank.Li@nxp.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>