PCI: endpoint: pci-ep-msi: Fix NULL vs IS_ERR() check in pci_epf_write_msi_msg()

The pci_epc_get() function returns error pointers. It never returns NULL.
Update the check to match.

Fixes: 1c3b002 ("PCI: endpoint: Add RC-to-EP doorbell support using platform MSI controller")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Link: https://patch.msgid.link/aIzCdV8jyBeql-Oa@stanley.mountain

Author: Dan Carpenter

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

@@ -24,7 +24,7 @@ static void pci_epf_write_msi_msg(struct msi_desc *desc, struct msi_msg *msg)
 	struct pci_epf *epf;
 
 	epc = pci_epc_get(dev_name(msi_desc_to_dev(desc)));
-	if (!epc)
+	if (IS_ERR(epc))
 		return;
 
 	epf = list_first_entry_or_null(&epc->pci_epf, struct pci_epf, list);