scsi: qla2xxx: Fix panic in qla_dfs_tgt_counters_show

When trying to display tgt_counters in the debugfs, a panic can result.

There is no null check for qpair after it is assigned in the for-loop.
Unless vha->hw->queue_pair_map array is completely filled with entries, the
system will panic dereferencing a null pointer.

Signed-off-by: Bill Kuzeja <william.kuzeja@stratus.com>
Acked-by: Himanshu Madhani <hmadhani@marvell.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>