r8169: Fix fall-through warning for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding a break statement instead of letting the code fall
through to the next case.

Link: https://github.com/KSPP/linux/issues/115Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Acked-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://lore.kernel.org/r/20210528202327.GA39994@embeddedorSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/realtek/r8169_main.c

@@ -4115,6 +4115,7 @@ static unsigned int rtl_quirk_packet_padto(struct rtl8169_private *tp,
 	case RTL_GIGA_MAC_VER_61:
 	case RTL_GIGA_MAC_VER_63:
 		padto = max_t(unsigned int, padto, ETH_ZLEN);
+		break;
 	default:
 		break;
 	}