gpio: pl061: Warn when IRQ line has not been configured

Existing (irq < 0) condition is always false because adev->irq has unsigned
type and contains 0 in case of failed irq_of_parse_and_map(). Up to now all
the mapping errors were silently ignored.

Seems that repairing this check would be backwards-incompatible and might
break the probe() for the implementations without IRQ support. Therefore
warn the user instead.

Signed-off-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>