iio: adc: vf610: Propagate the real error when platform_get_irq() fails

There is no need to pass a 'fake' return value when platform_get_irq() fails.

Propagate the real error instead.

While at it, only consider negative numbers returned by platform_get_irq()
as error.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>