hw/smc91c111: Add explicit 'return' rather than relying on fallthrough

Add an explicit 'return' statement to a case in smc91c111_readb
rather than relying on fallthrough to the following case's
return statement, for code clarity and to placate static analysers.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

hw/smc91c111.c

@@ -442,6 +442,7 @@ static void smc91c111_writeb(void *opaque, hwaddr offset,
             return;
         case 12: /* Early receive.  */
             s->ercv = value & 0x1f;
+            return;
         case 13:
             /* Ignore.  */
             return;