[Audioinjector] Fix bit offsets for equal volume and add 8 kHz operation (#1727)

Applying to the audioinjector sound card only. This patch offsets channel
2 correctly from the LR clock. This ensures that channel 2 doesn't loose
any bits during capture. It also results in both channels 1 and 2 having
the same volume. This commit also adds 8 kHz operation.

Signed-off-by: Matt Flax <flatmax@flatmax.org>