This fixes the count of Adv monitor upon monitor removal.

The following test was performed.
- Start two btmgmt consoles, issue a btmgmt advmon-remove command on one
console and observe a MGMT_EV_ADV_MONITOR_REMOVED event on the other.
Signed-off-by: Miao-chen Chou <mcchou@chromium.org>
Signed-off-by: Howard Chung <howardchung@google.com>
Reviewed-by: Alain Michaud <alainm@chromium.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>