gdbstub: mark expected switch fall-throughs

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

This patch fixes the following warnings:

kernel/debug/gdbstub.c: In function ‘gdb_serial_stub’:
kernel/debug/gdbstub.c:1031:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
    if (remcom_in_buffer[1] == '\0') {
       ^
kernel/debug/gdbstub.c:1036:3: note: here
   case 'C': /* Exception passing */
   ^~~~
kernel/debug/gdbstub.c:1040:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
    if (tmp == 0)
       ^
kernel/debug/gdbstub.c:1043:3: note: here
   case 'c': /* Continue packet */
   ^~~~
kernel/debug/gdbstub.c:1050:4: warning: this statement may fall through [-Wimplicit-fallthrough=]
    dbg_activate_sw_breakpoints();
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
kernel/debug/gdbstub.c:1052:3: note: here
   default:
   ^~~~~~~

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>
Acked-by: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>

kernel/debug/gdbstub.c

@@ -1033,13 +1033,14 @@ int gdb_serial_stub(struct kgdb_state *ks)
 				return DBG_PASS_EVENT;
 			}
 #endif
+			/* Fall through */
 		case 'C': /* Exception passing */
 			tmp = gdb_cmd_exception_pass(ks);
 			if (tmp > 0)
 				goto default_handle;
 			if (tmp == 0)
 				break;
-			/* Fall through on tmp < 0 */
+			/* Fall through - on tmp < 0 */
 		case 'c': /* Continue packet */
 		case 's': /* Single step packet */
 			if (kgdb_contthread && kgdb_contthread != current) {
@@ -1048,7 +1049,7 @@ int gdb_serial_stub(struct kgdb_state *ks)
 				break;
 			}
 			dbg_activate_sw_breakpoints();
-			/* Fall through to default processing */
+			/* Fall through - to default processing */
 		default:
 default_handle:
 			error = kgdb_arch_handle_exception(ks->ex_vector,