scsi: Simplify scsi_read/scsi_write()

There is no reason to directly point to static allocated array
when we have proper block_dev pointer available via parameter
in !CONFIG_BLK. For CONFIG_BLK this is read directly from uclass
platdata.

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