MN10300: Default config choice GDBSTUB_TTYSM0 should be GDBSTUB_ON_TTYSM0

The configuration choice for the port on which the GDB stub listens has
a default of GDBSTUB_TTYSM0, but this should be GDBSTUB_ON_TTYSM0 to
match the option.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>