phy: qcom-qmp-combo: clean up serdes initialisation

Clean up serdes initialisation somewhat by making the current helper a
dedicated helper for the DP part of the PHY.

Note that no error is currently returned for non-supported link rates.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Link: https://lore.kernel.org/r/20221114110621.4639-12-johan+linaro@kernel.org


Signed-off-by: Vinod Koul <vkoul@kernel.org>