selftests: pci_endpoint: Add GET_IRQTYPE checks to each interrupt test

Add GET_IRQTYPE API checks to each interrupt test.

While at it, change pci_ep_ioctl() to get the appropriate return
value from ioctl().

Suggested-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Link: https://lore.kernel.org/r/20250225110252.28866-2-hayashi.kunihiko@socionext.com


[kwilczynski: commit log]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>