Bluetooth: mgmt: Fix MGMT add advmon with RSSI command

The MGMT command: MGMT_OP_ADD_ADV_PATTERNS_MONITOR_RSSI uses variable
length argument. This causes host not able to register advmon with rssi.

This patch has been locally tested by adding monitor with rssi via
btmgmt on a kernel 6.1 machine.

Reviewed-by: Archie Pusaka <apusaka@chromium.org>
Fixes: b338d917

 ("Bluetooth: Implement support for Mesh")
Signed-off-by: Howard Chung <howardchung@google.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>