[SCSI] libiscsi: Use SCSI_MLQUEUE_TARGET_BUSY

For the conditions below we do not want the queuecommand
function to call us right back, so return SCSI_MLQUEUE_TARGET_BUSY.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>