scsi: qla2xxx: Make qla2x00_mem_free() easier to verify

Instead of clearing all freed pointers at the end of qla2x00_mem_free(),
clear freed pointers immediately after having freed the memory these
pointers point at.

Cc: Himanshu Madhani <hmadhani@marvell.com>
Cc: Giridhar Malavali <gmalavali@marvell.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Acked-by: Himanshu Madhani <hmadhani@marvell.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>