dvbapi: Check at least 8 adapters when detecting api.

Hi,
I have removed one (dual tuner) adapter from my pc.
Since I use fixed adapter numbers, I ended up with:
/dev/dvb/adapter2
/dev/dvb/adapter3

Then dvbapi stopped working because when detecting the api, it was only search for /dev/dvb/adapter0

This patch make the detection search at least the presence of 8 adapters.
(tested and working)