Bluetooth: HFP_HF: Add a function to reject the call

Add a function `bt_hfp_hf_reject` to reject the incoming call.

Add a flag BT_HFP_HF_FLAG_INCOMING to identify the incoming call.
Set the flag if the call_setup value is 1.

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