PCI: endpoint: Return error if EPC is started/stopped multiple times

When the EPC is started or stopped multiple times from configfs, just
return -EALREADY. There is no need to call the EPC start/stop functions
in those cases.

Link: https://lore.kernel.org/r/20230602114756.36586-4-manivannan.sadhasivam@linaro.org


Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Kishon Vijay Abraham I <kishon@kernel.org>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>