don't check platform_get_irq's return value against zero

platform_get_irq returns -ENXIO on failure, so !int_irq was probably
always true.  Better use (int)int_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.

Acked-by: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>