iio: adc: stm32-adc: Use generic_handle_domain_irq()

The call chain
	generic_handle_irq(irq_find_mapping(domain, x));

could be replaced with
	generic_handle_domain_irq(domain, x);

which looks up the struct irq_desc for the interrupt and handles it with
handle_irq_desc().
This is a slight optimisation given that the driver invokes only one
function and the struct irq_desc is used directly instead being looked
up via irq_to_desc().

Use generic_handle_domain_irq().

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/r/YnuYoQIzJoFIyEJY@linutronix.de