ASoC: msm8998: Set headphones only for a "3-pole-jack".

Set the MBHC config headphones_only flag when the
qcom,mbhc-audio-jack-type device tree property is "3-pole-jack".

Bug: 70920213
Test: Headphones only flag is set when the DTS contains:
      qcom,mbhc-audio-jack-type = "3-pole-jack";
Change-Id: Ic4a2ca6697fc2f7df453764f5ca03c814f27bc2a