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

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.