Bluetooth: HFP_HF: Add a function to terminate the call

Add a function `bt_hfp_hf_terminate` to terminate the active call.

Add a flag BT_HFP_HF_FLAG_ACTIVE to identify the call is active. Set
the flag if the call value is 1.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>