Mark switch cases where we are expecting to fall through.

This patch fixes the following warning (Building: arm):

drivers/net/ethernet/smsc/smc911x.c: In function ‘smc911x_phy_detect’:
drivers/net/ethernet/smsc/smc911x.c:677:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
    if (cfg & HW_CFG_EXT_PHY_DET_) {
       ^
drivers/net/ethernet/smsc/smc911x.c:715:3: note: here
   default:
   ^~~~~~~
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>