irqchip/mbigen: Free msi_desc on device teardown

Using irq_domain_free_irqs_common() on the irqdomain free path will
leave the MSI descriptor unfreed when platform devices get removed.
Properly free it by MSI domain free function.

Fixes: 9650c60e ("irqchip/mbigen: Create irq domain for each mbigen device")
Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200408114352.1604-1-yuzenghui@huawei.com