shared/btp: Fix return type in btp_send

This function returns bool.
diff --git a/src/shared/btp.c b/src/shared/btp.c
index 93df37a..7eea241 100644
--- a/src/shared/btp.c
+++ b/src/shared/btp.c
@@ -300,7 +300,7 @@
 	len = sizeof(*hdr) + length;
 	hdr = l_malloc(len);
 	if (!hdr)
-		return NULL;
+		return false;
 
 	hdr->service = service;
 	hdr->opcode = opcode;