net/ethernet: sc92031 is not Realtek

While the SC92031 could be found on fake "Realtek" NICs, it has no
relationship to Realtek, and is actually from Silan.

Create a new subdirectory for silan and move sc92031 there.

Signed-off-by: Cesar Eduardo Barros <cesarb@cesarb.net>
Signed-off-by: David S. Miller <davem@davemloft.net>