probe_irq_off() returns '0' on failure, not NO_IRQ, so the check
in this driver is clearly wrong. This replaces it with the
regular '!irq' check used in other drivers.

The sa1100 platform that this driver is used on originally numbered
all its interrupts starting at '0', which would have conflicted with
this change, but as of commit 18f3aec3 ("ARM: 8230/1: sa1100: shift
IRQs by one"), this is not a problem any more.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>