PCI: endpoint: pci-epf-vntb: Fix sparse build warning for epf_db

Use epf_db[i] dereference instead of readl() because epf_db is
in memory allocated by dma_alloc_coherent(), not I/O.

Remove useless/duplicated readl() in the process.

Link: https://lore.kernel.org/r/20221102141014.1025893-7-Frank.Li@nxp.comSigned-off-by: NFrank Li <frank.li@nxp.com>
Signed-off-by: NLorenzo Pieralisi <lpieralisi@kernel.org>

drivers/pci/endpoint/functions/pci-epf-vntb.c

@@ -136,7 +136,7 @@ struct epf_ntb {
 
 	struct epf_ntb_ctrl *reg;
 
-	void __iomem *epf_db;
+	u32 *epf_db;
 
 	phys_addr_t vpci_mw_phy[MAX_MW];
 	void __iomem *vpci_mw_addr[MAX_MW];
@@ -257,12 +257,10 @@ static void epf_ntb_cmd_handler(struct work_struct *work)
 	ntb = container_of(work, struct epf_ntb, cmd_handler.work);
 
 	for (i = 1; i < ntb->db_count; i++) {
-		if (readl(ntb->epf_db + i * sizeof(u32))) {
-			if (readl(ntb->epf_db + i * sizeof(u32)))
-				ntb->db |= 1 << (i - 1);
-
+		if (ntb->epf_db[i]) {
+			ntb->db |= 1 << (i - 1);
 			ntb_db_event(&ntb->ntb, i);
-			writel(0, ntb->epf_db + i * sizeof(u32));
+			ntb->epf_db[i] = 0;
 		}
 	}