PCI: endpoint: Use INTX instead of legacy

In the endpoint controller core code, change references to "legacy"
interrupts to "INTX" interrupts to match the term used in the PCI
specifications.

Link: https://lore.kernel.org/r/20231122060406.14695-4-dlemoal@kernel.org


Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>