mfd: pm8921: Fixup probe() error path when irq invalid

platform_get_irq() returns a negative error code when an IRQ is invalid
or unspecified.  Make 'irq' signed to properly handle this.

Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Josh Cartwright <joshc@codeaurora.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>