kgdb: Drop malformed kernel doc comment

Kernel doc does not understand POD variables to be referred to.

.../debug_core.c:73: warning: cannot understand function prototype:
'int                             kgdb_connected; '

Convert kernel doc to pure comment.

Fixes: dc7d5527

 ("kgdb: core")
Cc: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>