[SCSI] iscsi_transport: Added error status code for ping comp event

Defined error codes for ping completion status.

This patch take care of Mike Christie's commets

Signed-off-by: Vikas Chaudhary <vikas.chaudhary@qlogic.com>
Reviewed-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>