Bluetooth: Mesh: correct the return type

CC:  mesh/access.c
mesh/access.c: In function 'model_has_dst':
mesh/access.c:483:10:
	warning: returning 'u16_t *' {aka 'short unsigned int *'} from
	a function with return type '_Bool8' {aka 'unsigned char'} makes
	integer from pointer without a cast [-Wint-conversion]
  483 |   return bt_mesh_model_find_group(&mod, dst);
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: chao an <anchao@xiaomi.com>