net: sun8i-emac: annotate fallthrough

The Allwinner sun8i EMAC driver uses an implicit switch/case fallthrough
when setting up the MAC/PHY communication protocol, to handle the case
when RMII is requested, but would not be supported by the hardware.

Add our "fallthrough;" statement-like macro before the default branch in
sun8i_emac_set_syscon(), to avoid a warning when GCC's
-Wimplicit-fallthrough warning option is enabled.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>

drivers/net/sun8i_emac.c

@@ -335,6 +335,7 @@ static int sun8i_emac_set_syscon(struct sun8i_eth_pdata *pdata,
 			reg |= SC_RMII_EN | SC_ETCS_EXT_GMII;
 			break;
 		}
+		fallthrough;
 	default:
 		debug("%s: Invalid PHY interface\n", __func__);
 		return -EINVAL;