When the TEST UNIT READY command receives an error response, the
shutdown of the command's block data interface will result in
scsidev_ready() closing the SCSI device.  This will subsequently
result in a duplicate call to scsicmd_close(), leading to an assertion
failure when list_del() is called for the second time.

Fix by removing the command from the list of outstanding commands
before shutting down the command's interfaces.

Signed-off-by: Michael Brown <mcb30@ipxe.org>