[media] dvb-frontends/stv0367: refactor defaults table handling

Change defaults table writing so tables can be of dynamic length without
having to keep track of their lengths by adding and evaluating an end
marker (reg 0x0000), also move table writing to a dedicated function to
remove code duplication. Additionally mark st_register tables const since
they're used read-only.

Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>