Mark switch cases where we are expecting to fall through.

This patch fixes the following warning (Building: i386):

drivers/net/hamradio/baycom_epp.c: In function ‘transmit’:
drivers/net/hamradio/baycom_epp.c:491:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
    if (i) {
       ^
drivers/net/hamradio/baycom_epp.c:504:3: note: here
   default:  /* fall through */
   ^~~~~~~

Notice that, in this particular case, the code comment is
modified in accordance with what GCC is expecting to find.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>