scsi: Separate SCSI private block description initialization

When blk_create_device() is called some parameters in blk_desc are
automatically filled. Separate SCSI private initialization and SCSI full
block device initialization not to rewrite already prepared data.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>