Bluetooth: Fix memory leak in read_adv_mon_features()

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=f7f6e564f4202d8601c6


Signed-off-by: Peilin Ye <yepeilin.cs@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>