Bluetooth: HFP: Fix session pointer invalid issue

The dlc->session is invalid in the callback
hfp_hf_disconnected. The behavior of obtaining
an ACL connection handle through an invalid
pointer is unknown.

Add a field "struct bt_conn *acl" to "struct
bt_hfp_hf". Save acl connect handle to "struct
bt_hfp_hf::acl" in function bt_hfp_hf_accept.

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