[PATCH] pktcdvd: Correctly set cmd_len field in pkt_generic_packet

Fixes http://bugzilla.kernel.org/show_bug.cgi?id=7810

 - a silly
copy-paste bug introduced by the latest change.

Signed-off-by: Gerhard Dirschl <gd@spherenet.de>
Cc: Peter Osterlund <petero2@telia.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>