read_adv_mon_features() is leaking memory. Free `rp` before returning.

Fixes: e5e1e7fd ("Bluetooth: Add handler of MGMT_OP_READ_ADV_MONITOR_FEATURES")
Reported-and-tested-by: syzbot+f7f6e564f4202d8601c6@syzkaller.appspotmail.com
Link: https://syzkaller.appspot.com/bug?extid=f7f6e564f4202d8601c6Signed-off-by: NPeilin Ye <yepeilin.cs@gmail.com>
Signed-off-by: NMarcel Holtmann <marcel@holtmann.org>