[tr064] Wrong item-type for channel dslStatus (Switch instead of String)

According to the documentation of the tr-064 binding, the WAN subdevice channel dslStatus item-type is a string. On the contrary, in the UI (OH 4.3.2) it is indicated as a switch. The consequence of this mismatch is that when a link to an item is created, only a switch item can be created, which always remains UNDEF. The workaround is to modify the item type to string at a later time.

@J-N-K I raised an issue here