dm: scsi: Add operations for SCSI devices

The SCSI uclass currently has no operations. It just uses the global SCSI
functions. Fix this by adding operations to the only two drivers that use
the uclass, and replacing the global functions with those defined locally
in the SCSI code.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>