misc: pci_endpoint_test: Use INTX instead of LEGACY

In the root complex pci endpoint test function driver, change macros and
functions names using the term "legacy" to use "intx" instead to
match the term used in the PCI specifications.

Link: https://lore.kernel.org/r/20231122060406.14695-6-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>