PCI/MSI: Use irq_get_msi_desc() to simplify code

Use irq_get_msi_desc() to get MSI IRQ related msi_desc directly instead of
searching the dev->msi_list.

Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>