misc: pci_endpoint_test: Gate doorbell test on dynamic inbound mapping

misc: pci_endpoint_test: Gate doorbell test on dynamic inbound mapping

The doorbell test relies on the endpoint being able to update inbound
translations at runtime so the host can reach the doorbell target
through a BAR mapping.

If the endpoint does not advertise CAP_DYNAMIC_INBOUND_MAPPING, return
-EOPNOTSUPP from PCITEST_DOORBELL.

This avoids confusing failures in user space and kselftests when the
required capability is not available.

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/20260215150334.3391943-3-den@valinux.co.jp