platform_get_irq returns -ENXIO on failure, so !irq was probably
always true.  Better use irq <= 0.  Note that a return value of
zero is still handled as error even though this could mean irq0.

This is a followup to 305b3228 that
changed the return value of platform_get_irq from 0 to -ENXIO on error.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: David Vrabel <dvrabel@arcom.com>
Cc: Greg Kroah-Hartman <gregkh@suse.de>
Cc: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>