selftests: pci_endpoint: Skip IRQ test if IRQ is out of range.

The pci_endpoint_test tests the entire MSI/MSI-X range, which generates
false errors on platforms that do not support the whole range.

Skip the test in such cases and report accordingly.

Signed-off-by: Christian Bruel <christian.bruel@foss.st.com>
[mani: reworded description]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Link: https://patch.msgid.link/20250804170916.3212221-4-christian.bruel@foss.st.com