No need to return a 'fake' return value on platform_get_irq() failure.

Just return the error code itself instead.

Also, change the error condition to irq < 0, so that only negative values
are treated as errors.
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Tejun Heo <tj@kernel.org>