greybus: Use fallthrough pseudo-keyword

Replace the existing /* fall through */ comments and its variants with
the new pseudo-keyword macro fallthrough[1].

[1] https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-throughSigned-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20200727183258.GA28571@embeddedorSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/greybus/es2.c

@@ -759,7 +759,7 @@ static int check_urb_status(struct urb *urb)
 	case -EOVERFLOW:
 		dev_err(dev, "%s: overflow actual length is %d\n",
 			__func__, urb->actual_length);
-		/* fall through */
+		fallthrough;
 	case -ECONNRESET:
 	case -ENOENT:
 	case -ESHUTDOWN:

drivers/greybus/interface.c

@@ -1233,7 +1233,7 @@ int gb_interface_add(struct gb_interface *intf)
 	case GB_INTERFACE_TYPE_GREYBUS:
 		dev_info(&intf->dev, "GMP VID=0x%08x, PID=0x%08x\n",
 			 intf->vendor_id, intf->product_id);
-		/* fall-through */
+		fallthrough;
 	case GB_INTERFACE_TYPE_UNIPRO:
 		dev_info(&intf->dev, "DDBL1 Manufacturer=0x%08x, Product=0x%08x\n",
 			 intf->ddbl1_manufacturer_id,