maiblox: mediatek: Fix handling of platform_get_irq() error

platform_get_irq() returns -ERRNO on error.  In such case casting to u32
and comparing to 0 would pass the check.

Fixes: 623a6143 ("mailbox: mediatek: Add Mediatek CMDQ driver")
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>