se: Fix Wbool-conversion warning in find_specific_rule()

find_specific_rule() returns pointer, so obviously 'return false' is
just confusingly stupid.  Newer clang (e.g. on ArchLinux) also
complains:

  se/ace.c:661:10: error: initialization of pointer of type 'struct seel_ace_rule *' to null from a constant boolean expression [-Werror,-Wbool-conversion]

Fixes: 1be05e6 ("se: Initial commit")
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>

Author: krzk

se/ace.c

@@ -658,7 +658,7 @@ static struct seel_ace_rule *find_specific_rule(struct seel_ace *ace,
 	DBG("%zu", aid_len);
 
 	if (!hash || !aid)
-		return false;
+		return NULL;
 
 	for (list = ace->rules; list; list = list->next) {
 		struct seel_ace_rule *rule = list->data;