mmc: sunxi: Avoid #ifdefs in delay and width setup

The delay and bus-width setup are slightly different across the
Allwinner SoC generations, and we covered this so far with some
preprocessor conditionals.

Use the more readable IS_ENABLE() instead.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>