Bluetooth: MGMT: Fix possible crash on mgmt_index_removed

Bluetooth: MGMT: Fix possible crash on mgmt_index_removed

If mgmt_index_removed is called while there are commands queued on
cmd_sync it could lead to crashes like the bellow trace:

0x0000053D: __list_del_entry_valid_or_report+0x98/0xdc
0x0000053D: mgmt_pending_remove+0x18/0x58 [bluetooth]
0x0000053E: mgmt_remove_adv_monitor_complete+0x80/0x108 [bluetooth]
0x0000053E: hci_cmd_sync_work+0xbc/0x164 [bluetooth]

So while handling mgmt_index_removed this attempts to dequeue
commands passed as user_data to cmd_sync.

Fixes: 7cf5c2978f23 ("Bluetooth: hci_sync: Refactor remove Adv Monitor")
Reported-by: jiaymao <[email protected]>
Signed-off-by: Luiz Augusto von Dentz <[email protected]>