cuda: fix reset-all and power-off words

There is a long-standing bug in the CUDA implementation of the reset-all and
power-off words whereby an extra byte is written after the CUDA_RESET_SYSTEM
and CUDA_POWERDOWN commands.

This extra byte used to be ignored in QEMU until commits 017da0b568 ("cuda:
port POWERDOWN command to new framework") and 54e894442e ("cuda: port
RESET_SYSTEM command to new framework") added a check which rejects the
command if the command length is incorrect.

Fix the outgoing command length to remove the extra byte which allows the
reset-all and power-off words to work in QEMU once again.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Hervé Poussineau <hpoussin@reactos.org>
Fixes: qemu#624