pcie: msi: fix MSI-X fallback to MSI

When enabling MSI & MSI-X, the code seemed to handle fallback to MSI
when MSI-X is not available, but the logic uses MSI-X even if not
available and the MSI path never gets used.

Fixes: a2491b32 ("drivers/pcie: Add support for MSI-X")
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>