sunxi: mmc: Add 'sunxi_' prefix to the static functions

This results in a much more readable callgraph, because now they
can't be confused with the function having exactly the same name
in the generic mmc code.

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>