samples/damon/mtier: avoid starting DAMON before initialization

Commit 9643143 ("samples/damon/mtier: support boot time enable
setup") is somehow incompletely applying the origin patch [1].  It is
missing the part that avoids starting DAMON before module initialization. 
Probably a mistake during a merge has happened.  Fix it by applying the
missed part again.

Link: https://lkml.kernel.org/r/20250909022238.2989-4-sj@kernel.org
Link: https://lore.kernel.org/20250706193207.39810-4-sj@kernel.org [1]
Fixes: 9643143 ("samples/damon/mtier: support boot time enable setup")
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: sjp38

samples/damon/mtier.c

@@ -208,6 +208,9 @@ static int damon_sample_mtier_enable_store(
 	if (enabled == is_enabled)
 		return 0;
 
+	if (!init_called)
+		return 0;
+
 	if (enabled) {
 		err = damon_sample_mtier_start();
 		if (err)