dm: mmc: sunxi: Pass private data around explicitly

At present the driver-private data is obtained in various functions by
various means. With driver model this is provided automatically. Without
driver model it comes from a C array declared at the top of the file.

Adjust internal functions so that they are passed the private data as
a parameter, allowing the caller to obtain it using either means.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>