iio: adc: meson_saradc: Fix memory allocation order

meson_saradc's irq handler uses priv->regmap so make sure that it is
allocated before the irq get enabled.

This also fixes crash when CONFIG_DEBUG_SHIRQ is enabled, as device
managed resources are freed in the inverted order they had been
allocated, priv->regmap was freed before the spurious fake irq that
CONFIG_DEBUG_SHIRQ adds called the handler.

Fixes: 3af10913 ("iio: adc: meson-saradc: switch from polling to interrupt mode")
Reported-by: Elie Roudninski <xademax@gmail.com>
Signed-off-by: Remi Pommarel <repk@triplefau.lt>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Tested-by: Elie ROUDNINSKI <xademax@gmail.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>