staging:iio:spear_adc: Fix IRQ check

The test in the spear_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: Stefan Roese <sr@denx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>