char: lp: 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 warnings:

drivers/char/lp.c: In function ‘lp_compat_ioctl’:
drivers/char/lp.c:756:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (!COMPAT_USE_64BIT_TIME) {
      ^
drivers/char/lp.c:761:2: note: here
  case LPSETTIMEOUT_NEW:
  ^~~~
drivers/char/lp.c: In function ‘lp_ioctl’:
drivers/char/lp.c:728:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (BITS_PER_LONG == 32) {
      ^
drivers/char/lp.c:733:2: note: here
  case LPSETTIMEOUT_NEW:
  ^~~~

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

Notice that in some cases, 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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>