ASoC: wcd9310: Don't overwrite detected unsupported headset

The current US/EURO headset detection algorithm is overwriting detected
unsupported headset detection with invalid headset detection.
Don't make it to overwrite to report unsupported headset correctly.

CRs-fixed: 359290, 368319
Change-Id: If2d02c0d68be1c6f3f2eb1aa89c7a08ffe166446
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>