audio: hal: fix click "resume call" two times will cause no sound in HFP call.

If click "resume call" many times when making hfp call, telecom will send
REQUEST_UNHOLD and call start_hfp in audio hal many times, then pcm_open
failed, and no sound will be heard when we make hfp call again. So return 0
at the beginning of start_hfp if hfp is already active.

Bug: 33226115
Bug: 33459120

Change-Id: I517918ef55fce96cf3761e04db89ed634250165f