mmc: sunxi: Fix NULL pointer reference on clk_delays

Some SoCs do not support clk delays for MMC in the clock control unit.
These include the old controllers in A10/A10s/A13/R8, and the new eMMC
controller in A64. The config structure for these controllers do not
specify clk_delays, but the check for this was replaced in change
"mmc: sunxi: Support controllers that can use both old and new timings".

This patch adds back the check for clk_delays, and also adds comments
for both checks in sunxi_mmc_clk_set_phase().

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