ASoC: Handle low measured DC offsets for wm_hubs devices

commit 20a4e7fc upstream.

The DC servo codes are actually signed numbers so need to be treated as
such.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>