usb: s3c-hsotg: Fix maximum patcket size setting for EP0

MPS field of DOEPCTL0 is read only.

Signed-off-by: Anton Tikhomirov <av.tikhomirov@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
diff --git a/drivers/usb/gadget/s3c-hsotg.c b/drivers/usb/gadget/s3c-hsotg.c
index e3fe5fd..03de169 100644
--- a/drivers/usb/gadget/s3c-hsotg.c
+++ b/drivers/usb/gadget/s3c-hsotg.c
@@ -1696,10 +1696,12 @@
 	reg |= mpsval;
 	writel(reg, regs + S3C_DIEPCTL(ep));
 
-	reg = readl(regs + S3C_DOEPCTL(ep));
-	reg &= ~S3C_DxEPCTL_MPS_MASK;
-	reg |= mpsval;
-	writel(reg, regs + S3C_DOEPCTL(ep));
+	if (ep) {
+		reg = readl(regs + S3C_DOEPCTL(ep));
+		reg &= ~S3C_DxEPCTL_MPS_MASK;
+		reg |= mpsval;
+		writel(reg, regs + S3C_DOEPCTL(ep));
+	}
 
 	return;