genirq/irqdomain: Remove redundant NULL pointer check on fwnode

The is_fwnode_irqchip() helper will check if the fwnode_handle is empty.
There is no need to perform a redundant check outside of it.

Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200716083905.287-1-yuzenghui@huawei.com