irqchip/mbigen: Don't clear eventid when freeing an MSI

mbigen_write_msg clears eventid bits of a mbigen register
when free a interrupt, because msi_domain_deactivate memset
struct msg to zero. Then multiple mbigen pins with zero eventid
will report the same interrupt number.

The eventid clear call trace:
                free_irq
                __free_irq
                irq_shutdown
                irq_domain_deactivate_irq
                __irq_domain_deactivate_irq
                __irq_domain_deactivate_irq
                msi_domain_deactivate
                platform_msi_write_msg
                mbigen_write_msg

Signed-off-by: Jianguo Chen <chenjianguo3@huawei.com>
[maz: massaged subject]
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>