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: 7cf5c297 ("Bluetooth: hci_sync: Refactor remove Adv Monitor")
Reported-by: jiaymao <quic_jiaymao@quicinc.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>