Fix #1545 - ensure files only created as required in on-demand mode.
dvb: undo mod to frequency range testing and add additional tuning check

The previous check was garbage because for -S adapters the frequency range
reported by the adapter is in local units (i.e. hi/low band taken into
account) and not simple kHz. This makes checking more difficult and while
its possible I think this should be left for dvb rewrite.

Instead I've re-instated the old code, but muxes are only disable if the
return is EINVAL, which should only be returned if the DVB layer decides
the params are out of range.