irqchip/riscv-imsic: Fix boot time update effective affinity warning

Currently, the following warning is observed on the QEMU virt machine:
genirq: irq_chip APLIC-MSI-d000000.aplic did not update eff. affinity mask of irq 12

The above warning is because the IMSIC driver does not set the initial
value of effective affinity in the interrupt descriptor. To address this,
initialize the effective affinity in imsic_irq_domain_alloc().

Fixes: 027e125a ("irqchip/riscv-imsic: Add device MSI domain support for platform devices")
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240413065210.315896-1-apatel@ventanamicro.com