staging:iio:lpc32xx_adc: Fix IRQ check

The test in the lpc32xx_adc driver which checks whether the IRQ number returned
by platform_get_irq() has multiple problems. It accepts 0 even though this is an
invalid IRQ. It also rejects IRQ numbers that are larger or equal than NR_IRQS.
First of all drivers should never need to reference NR_IRQS and secondly with
CONFIG_SPARSE_IRQ NR_IRQS is not the upper limit, so the check might reject
valid IRQ numbers. This patch modifies the check to only test against less or
equal to 0.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Cc: Roland Stigge <stigge@antcom.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>