Bluetooth: hci_sync: Convert MGMT_OP_START_DISCOVERY

This make use of hci_cmd_sync_queue for MGMT_OP_START_DISCOVERY,
MGMT_OP_START_SERVICE_DISCOVERY and MGMT_OP_STOP_DISCOVERY to use
hci_cmd_sync_queue so they no longer depend on hdev->discov_update work
to send any commands.

Tested with:

tools/mgmt-tester -s "Start Discovery"

Test Summary
------------
Start Discovery - Not powered 1                      Passed
Start Discovery - Invalid parameters 1               Passed
Start Discovery - Not supported 1                    Passed
Start Discovery - Success 1                          Passed
Start Discovery - Success 2                          Passed
Start Discovery - Power Off 1                        Passed
Start Discovery BREDR LE - (Ext Scan Enable)         Passed
Start Discovery LE - (Ext Scan Enable)               Passed
Start Discovery LE - (Ext Scan Param)                Passed
Start Discovery - (2m, Scan Param)                   Passed
Start Discovery - (coded, Scan Param)                Passed
Start Discovery - (1m, 2m, coded, Scan Param)        Passed
LL Privacy - Start Discovery 1 (Disable RL)          Passed
LL Privacy - Start Discovery 2 (Disable RL)          Passed
Total: 14, Passed: 14 (100.0%), Failed: 0, Not Run: 0

tools/mgmt-tester -s "Start Service"

Test Summary
------------
Start Service Discovery - Not powered 1              Passed
Start Service Discovery - Invalid parameters 1       Passed
Start Service Discovery - Not supported 1            Passed
Start Service Discovery - Success 1                  Passed
Start Service Discovery - Success 2                  Passed
Total: 5, Passed: 5 (100.0%), Failed: 0, Not Run: 0

tools/mgmt-tester -s "Stop Discovery"

Test Summary
------------
Stop Discovery - Success 1                           Passed
Stop Discovery - BR/EDR (Inquiry) Success 1          Passed
Stop Discovery - Rejected 1                          Passed
Stop Discovery - Invalid parameters 1                Passed
Stop Discovery - (Ext Scan Disable)                  Passed
Total: 5, Passed: 5 (100.0%), Failed: 0, Not Run: 0

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>