scsi: dm: Unbind all scsi based block devices before new scan

New scan should unbind all block devices not to be listed again.
Without this patch if scsi reset or scan is called new block devices are
created which point to the same id and lun.

For example:
ZynqMP> scsi scan
scsi_scan: if_type=2, devnum=0: sdhci@ff170000.blk, 6, 0
scsi_scan: if_type=2, devnum=0: ahci@fd0c0000.id1lun0, 2, 0
scsi_scan: if_type=2, devnum=0: ahci@fd0c0000.id1lun0, 2, 1
scsi_scan: if_type=2, devnum=0: ahci@fd0c0000.id1lun0, 2, 2
scsi_scan: if_type=2, devnum=0: ahci@fd0c0000.id1lun0, 2, 3
scsi_scan: if_type=2, devnum=0: ahci@fd0c0000.id1lun0, 2, 4
scanning bus for devices...
  Device 0: (1:0) Vendor: ATA Prod.: KINGSTON SVP200S Rev: 501A
            Type: Hard Disk
            Capacity: 57241.8 MB = 55.9 GB (117231408 x 512)

Reported-by: Ken Ma <make@marvell.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Simon Glass <sjg@chromium.org>