Enable DC blocking filter on Spring.
Enable DC blocking filter on Spring.

Like Daisy before it, enable the DC blocking filter.  This sets bit 4
in register 0x18.

BUG=chrome-os-partner:21400
TEST=boot spring record wav and check for DC offset with audio editor

Change-Id: Ie0dbe8d4c5e1afb2362d83b254a234780b29759b
Signed-off-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/63668
Reviewed-by: Hsinyu Chao <8b91762bdbe27d3121b311091d7ca5125e9a8df6@chromium.org>
Tested-by: Chih-Chung Chang <ebe32a3bdce230d12f75781016b6b880113fb486@chromium.org>

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