sata_fsl: fix error handling of irq_of_parse_and_map

irq_of_parse_and_map() returns 0 on error (the result is unsigned int),
so testing for negative result never works.

Signed-off-by: Dmitry Torokhov <dtor@chromium.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org