PCI: rockchip-ep: Use INTX instead of legacy

Rename the function rockchip_pcie_ep_send_legacy_irq() of the rockchip
endpoint driver to rockchip_pcie_ep_send_intx_irq(). Uses of the term
"legacy" are also replaced with "INTX" in comments.

Link: https://lore.kernel.org/r/20231122060406.14695-15-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>