se: Fix Wbool-conversion warning in find_generic_rule_for_aid()

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

  se/ace.c:719:10: 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

@@ -716,7 +716,7 @@ static struct seel_ace_rule *find_generic_rule_for_aid(struct seel_ace *ace,
 	DBG("%zu", aid_len);
 
 	if (!aid)
-		return false;
+		return NULL;
 
 	for (list = ace->rules; list; list = list->next) {
 		struct seel_ace_rule *rule = list->data;