src.ctf.fs: fix switch cases using wrong enumerator

Fix these two cases using CTF_MSG_ITER_MEDIUM_STATUS_FOO instead of
CTF_MSG_ITER_STATUS_FOO.  This isn't a problem in practice, because they
have the same numerical values.

Change-Id: I5d9e22c4e25e9c28a488c4a7b12bf102208dacf0
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7095
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>