firedtv: use length_field() of PMT as length

Parsed and used the length_field() of the PMT message instead of using
the length field of the message struct, which does not seem to be filled
correctly by e.g. MythTV.

Signed-off-by: Henrik Kurelid <henrik@kurelid.se>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>