PCI: endpoint: Fix clearing start entry in configfs

After an endpoint is started through configfs, if 0 is written to the
configfs entry 'start', the controller stops but the epc_group->start
value remains 1.

A subsequent unlinking of the function from the controller would trigger
a spurious WARN_ON_ONCE() in pci_epc_epf_unlink() despite right
behavior.

Fix it by setting epc_group->start = 0 when a controller is stopped
using configfs.

Fixes: d746799116 ("PCI: endpoint: Introduce configfs entry for configuring EP functions")
Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>
Cc: Kishon Vijay Abraham I <kishon@ti.com>

drivers/pci/endpoint/pci-ep-cfs.c

@@ -57,6 +57,7 @@ static ssize_t pci_epc_start_store(struct config_item *item, const char *page,
 
 	if (!start) {
 		pci_epc_stop(epc);
+		epc_group->start = 0;
 		return len;
 	}