PCI: dwc: Fix uninitialized variable in dw_handle_msi_irq()

The bug is that "val" is unsigned long but we only initialize 32 bits of
it.  Then we test "if (val)" and that might be true not because we set the
bits but because some were never initialized.

Fixes: f342d940

 ("PCI: exynos: Add support for MSI")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>