examples/mi-mctp: fix warning for possibly uninitialised variable

examples/mi-mctp: fix warning for possibly uninitialised variable

gcc-12 with -Wuninitialized-var reports a warning for mi-mctp:

    examples/mi-mctp.c: In function ‘do_action_endpoint’:
    examples/mi-mctp.c:665:16: warning: ‘rc’ may be used uninitialized [-Wmaybe-uninitialized]
      665 |         return rc;
          |                ^~
    examples/mi-mctp.c:637:13: note: ‘rc’ was declared here
      637 |         int rc;
          |             ^~

We shouldn't be able to hit this path, but explicitly set rc to an error
if we miss an enum case.

Fixes: https://github.com/linux-nvme/libnvme/issues/467
Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>