PCI: endpoint: pci-epf-test: Fix doorbell test support

The doorbell feature temporarily overrides the inbound translation to point
to the address stored in epf_test->db_bar.phys_addr, i.e., it calls
set_bar() twice without ever calling clear_bar(), as calling clear_bar()
would clear the BAR's PCI address assigned by the host.

Thus, when disabling the doorbell, restore the inbound translation to point
to the memory allocated for the BAR.

Without this, running the PCI endpoint kselftest doorbell test case more
than once would fail.

Fixes: eff0c286 ("PCI: endpoint: pci-epf-test: Add doorbell test support")
Signed-off-by: Niklas Cassel <cassel@kernel.org>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Link: https://patch.msgid.link/20250908161942.534799-2-cassel@kernel.org