spi: pca2xx-pci: Allow MSI

Now that the core is ready for edge-triggered interrupts, we can safely
allow the PCI versions that provide this to enable the feature and,
thus, have less shared interrupts.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>