[media] DVB: dvb_frontend: fix dtv_property_dump for DTV_DVBT2_PLP_ID

- Add missing entry to array "dtv_cmds".
- Set array size to DTV_MAX_COMMAND + 1 to avoid future off-by-ones.

Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Andreas Oberritter <obi@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>