PCI/MSI: Add msi_setup_entry() to clean up MSI initialization

Move MSI entry stuff to a new function, msi_setup_entry(), to simplify
msi_capability_init() as MSI-X does.

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