PCI: cadence-ep: Fix the driver to send MSG TLP for INTx without data payload

PCI: cadence-ep: Fix the driver to send MSG TLP for INTx without data payload

Per the Cadence's "PCIe Controller IP for AX14" user guide, Version
1.04, Section 9.1.7.1, "AXI Subordinate to PCIe Address Translation
Registers", Table 9.4, the bit 16 of the AXI Subordinate Address
(axi_s_awaddr) when set corresponds to MSG with data, and when not set,
to MSG without data.

However, the driver is currently doing the opposite and due to this,
the INTx is never received on the host.

So, fix the driver to reflect the documentation and also make INTx work.

Fixes: 37dddf14f1ae ("PCI: cadence: Add EndPoint Controller driver for Cadence PCIe controller")
Signed-off-by: Hans Zhang <18255117159@163.com>
Signed-off-by: Hans Zhang <hans.zhang@cixtech.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://lore.kernel.org/r/20250214165724.184599-1-18255117159@163.com
[kwilczynski: commit log]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>