scsi: target: cxgbit: add support for IEEE_8021QAZ_APP_SEL_STREAM selector

IEEE_8021QAZ_APP_SEL_STREAM is a valid selector for iSCSI connections, so
add code to use IEEE_8021QAZ_APP_SEL_STREAM selector to get priority mask.

Signed-off-by: Varun Prakash <varun@chelsio.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>