dm: scsi: Separate the non-command code into its own file

At present the SCSI command code includes both the command-processing code
and the core SCSI functions and data structures.

Separate the latter into its own file, adding functions as needed to avoid
the command code accessing data structures directly. This functions use the
new legacy block functions.

With this commit:
- There is no CONFIG option referenced from the command code
- The concept of a 'current SCSI device' is confined to the command code

This will make it easier to convert this code to driver model.

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