[SCSI] qla4xxx: use iscsi class session state check ready

This has qla4xxx use the iscsi class's check ready function
in the queue command function, so all iscsi drivers return the
same error value for common problems.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Cc: David Somayajulu <david.somayajulu@qlogic.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>