mmc: sunxi: fix support for new timings mode only SoCs

The A83T MMC support code introduces the timings mode switch, however
such a switch doesn't exist on new SoCs with only new timings mode.

Only execute the switch if the SoC really have the timings mode switch,
to fix the regression shown on new timings mode only SoCs (A64, H5,
etc).

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>