iio:adc:berlin2-adc: pass up real error code

Pass up the real error code returned by platform_get_irq_byname().

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Acked-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>