mhi: pci_generic: Fix shared MSI vector support

When a shared MSI vector must be used (e.g. when VTd is disabled on
x86_64), each event MSI vector must be set to the shared vector idx.

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Hemant <Kumar&lt;hemantk@codeaurora.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>