scsi: qla2xxx: use dma_mapping_error to check map errors

The return value of dma_map_single() should be checked by
dma_mapping_error(). However, in function qla26xx_dport_diagnostics(), its
return value is checked against NULL, which could result in failures.

Signed-off-by: Pan Bian <bianpan2016@163.com>
Acked-by: Himanshu Madhani <himanshu.madhani@cavium.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>