[SCSI] qla4xxx: cleanup, make qla4xxx_build_ddb_list short

Make qla4xxx_build_ddb_list shorter by adding more helper functions.
Signed-off-by: Lalit Chandivade <lalit.chandivade@qlogic.com>
Signed-off-by: Vikas Chaudhary <vikas.chaudhary@qlogic.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>