scsi: qla2xxx: Add boundary checks for exchanges to be offloaded

Max boundary for exchange off load is 32k exchanges. If a system
is unable to allocate large memory buffer to support this feature,
then driver will reduce the number of exchanges down to a value
system can support.

Signed-off-by: Quinn Tran <quinn.tran@cavium.com>
Signed-off-by: Himanshu Madhani <himanshu.madhani@cavium.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>