media: hdpvr: add terminating 0 at end of string

dev->usbc_buf was passed as argument for %s, but it was not safeguarded
by a terminating 0.

This caused this syzbot issue:

https://syzkaller.appspot.com/bug?extid=79d18aac4bf1770dd050



Reported-and-tested-by:  <syzbot+79d18aac4bf1770dd050@syzkaller.appspotmail.com>

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>