selftests: pci_endpoint: Skip doorbell test when unsupported

selftests: pci_endpoint: Skip doorbell test when unsupported

PCITEST_DOORBELL may return -EOPNOTSUPP when the endpoint does not
advertise CAP_DYNAMIC_INBOUND_MAPPING.

Treat this like other optional capabilities and skip the doorbell test
instead of reporting a failure.

Suggested-by: Niklas Cassel <cassel@kernel.org>
Signed-off-by: Koichiro Den <den@valinux.co.jp>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Reviewed-by: Niklas Cassel <cassel@kernel.org>
Link: https://patch.msgid.link/20260215150334.3391943-4-den@valinux.co.jp