canopen: Fixed tree expansion

Previously the COB-ID and Type subtree also used the main ett_canopen
variable. This made control of subtree expansion impossible as both
parent and subtree nodes shared the same state.

Change-Id: I3dabf7f399e83bfcfbf78bc0e633e3696776c043
Reviewed-on: https://code.wireshark.org/review/5480
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>