PCI: dwc: Fix dw_pcie_free_msi() if msi_irq is invalid

Check msi_irq variable before calling irq_set_chained_handler() and
irq_set_handler_data(), lest we call those functions for an invalid MSI
IRQ.

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Gustavo Pimentel <gustavo.pimentel@synopsys.com>