dm: scsi: fix divide-by-0 error in scsi_scan()

With DM_SCSI enabled, blk_create_devicef() is called with blkz = 0, leading
to a divide-by-0 exception.
scsi_detect_dev() can be used to get the required parameters (block size
and number of blocks) from the drive before calling blk_create_devicef().

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>