davicom: Annotate implicit fall through in dm9000_set_io

There is a plan to build the kernel with -Wimplicit-fallthrough and
this place in the code produced a warning (W=1).

This commit removes the following warning:

  include/linux/device.h:1480:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
  drivers/net/ethernet/davicom/dm9000.c:397:3: note: in expansion of macro 'dev_dbg'
  drivers/net/ethernet/davicom/dm9000.c:398:2: note: here

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: David S. Miller <davem@davemloft.net>