ttusb-dec: buffer overflow in ioctl

commit f2e323ec96077642d397bb1c355def536d489d16 upstream.

We need to add a limit check here so we don't overflow the buffer.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
[bwh: Backported to 3.2: adjust filename]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>