net: sun8i-emac: annotate fallthrough

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>