misc: pci_endpoint_test: Use a unique test pattern for each BAR

Use a unique test pattern for each BAR in. This makes it easier to
detect/debug address translation issues, since a developer can dump
the backing memory on the EP side, using e.g. devmem, to verify that
the address translation for each BAR is actually correct.

Link: https://lore.kernel.org/linux-pci/20231215105952.1531683-1-nks@flawful.org
Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>