scsi: qla2xxx: Improve readability of the code that handles qla_flt_header

scsi: qla2xxx: Improve readability of the code that handles qla_flt_header

Declare qla_hw_data.flt as a qla_flt_header pointer instead of as a void
pointer. Add a zero-length array at the end of struct qla_flt_header to
make it clear that qla_flt_header and qla_flt_region are contiguous. This
patch removes several casts but does not change any functionality.

Cc: Himanshu Madhani <hmadhani@marvell.com>
Cc: Quinn Tran <qutran@marvell.com>
Cc: Martin Wilck <mwilck@suse.com>
Cc: Daniel Wagner <dwagner@suse.de>
Cc: Roman Bolshakov <r.bolshakov@yadro.com>
Link: https://lore.kernel.org/r/20191219004706.39039-1-bvanassche@acm.org
Reviewed-by: Daniel Wagner <dwagner@suse.de>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>