staging: gdm724x: 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/20200727190829.GA30332@embeddedorSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/gdm724x/gdm_lte.c

@@ -784,7 +784,7 @@ static int gdm_lte_receive_pkt(struct phy_dev *phy_dev, char *buf, int len)
 			return index;
 		dev = phy_dev->dev[index];
 		gdm_lte_pdn_table(dev, buf, len);
-		/* Fall through */
+		fallthrough;
 	default:
 		ret = gdm_lte_event_send(dev, buf, len);
 		break;