[pvr] don't reset internal group channel numbers if reset channel number cache

[xhaggi]

Should fix the issue reported in Trac http://trac.kodi.tv/ticket/15563

When "use backend channel number" is selected in settings/live tv/general (using pvr.wmc client) infolabel Listitem.ChannelNumberLabel returns labels in the form "0.y" (y = minor channel number for US ATSC virtual channels) for all channels which are not members of the selected channel group (when "use backend channel number" is not selected, the infolabel returns "0"). Observed in Confluence Dialog PVRGroupManager.
Expected result would either be "0" for channels not in current channel group or "x.y" with the actual major.minor virtual channel number. Assumed source is PVRChannelGroupInternal.cpp.

[opdenkamp]

I'm not really comfortable with changing this at this point. I know that it was needed for something when I wrote it, but it was ~4 years ago and I don't remember why exactly.

Since it's not fixing a critical bug, it should wait until the first point release.

[xhaggi]

jenkins build this please

[xhaggi]

@opdenkamp ping

[opdenkamp]

merge away, but as i said earlier, it was needed earlier, so if you run into issues when switching between groups, you know why ;-)