check if there are ISRCs at all

When no ISRCs are present (valid CRC or not) in the first 100 sectors
(150 to be safe), there are no ISRCs on the track at all
so we stop searching.

Otherwise we would search the whole track, which then takes like 5
minutes per disc.
define SECTORS_ISRC and read 10 more

Reading more sectors takes more time.
10 more would be 10% more, rather than 50%.
This should be fine.