drivers/block/pktcdvd.c: avoid useless memset

Avoid the 'memset(...,0, ...)' before calling 'init_cdrom_command' because
this function already does it.

Signed-off-by: Christophe Jaillet <jaillet.christophe@wanadoo.fr>
Acked-by: Peter Osterlund <petero2@telia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>