Most block devices provide a command (e.g. 'sata', 'scsi', 'ide') and
these commands generally do the same thing. This makes it harder to
maintain this code and keep it consistent.

We now have a block device interface which is either implemented by driver
model (when CONFIG_BLK is enabled) or with a legacy interface. Therefore
it is possible to handle most of what these commands do with generic code.

Add a new generic function to process block-device commands using the
interface type and the current device number for that type.

Signed-off-by: Simon Glass <sjg@chromium.org>