media: hdpvr: fix compiler warning

In function 'strncpy',
    inlined from 'vidioc_g_audio' at media-git/drivers/media/usb/hdpvr/hdpvr-video.c:876:2:
media-git/include/linux/string.h:246:9: warning: '__builtin_strncpy' specified bound 32 equals destination size [-Wstringop-truncation]
  return __builtin_strncpy(p, q, size);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>