Add the TX offset slot programming.  There is no RX offset slot
register.

Since there is no RX offset the check for slot symmetry can be removed.
Signed-off-by: Dan Murphy <dmurphy@ti.com>
Link: https://lore.kernel.org/r/20200709185129.10505-2-dmurphy@ti.comSigned-off-by: Mark Brown <broonie@kernel.org>