platform_get_irq() returns a negative error code when an IRQ is invalid
or unspecified.  Make 'irq' signed to properly handle this.
Reviewed-by: NStephen Boyd <sboyd@codeaurora.org>
Signed-off-by: NJosh Cartwright <joshc@codeaurora.org>
Signed-off-by: NLee Jones <lee.jones@linaro.org>