parport: ieee1284: mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch
cases where we are expecting to fall through.

This patch fixes the following warning:

drivers/parport/ieee1284.c: In function ‘parport_read’:
drivers/parport/ieee1284.c:722:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (parport_negotiate (port, IEEE1284_MODE_NIBBLE)) {
      ^
drivers/parport/ieee1284.c:726:2: note: here
  case IEEE1284_MODE_NIBBLE:
  ^~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

Notice that, in this particular case, the code comment is modified
in accordance with what GCC is expecting to find.

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>