Do not count transferred and received bytes twice in case of PPPoE

When setting the WAN primary connection to PPPoE, calling get_wan_unit with either ppp0 or its underlying interface will return 0. This may cause double counting of transferred and received bytes and may be the cause of showing wrong/weird numbers in the traffic monitor page. (I remember reading this investigation somewhere, but can't seem to find the forum thread.)

Instead of skipping ppp0 interface as proposed, I think it is better to use the ppp0 interface and skip the underlying interface.
To do this, I have added an extra check that compares the interface name with the WAN interface name using get_wan_ifname. If on the other hand you want to count traffic of the underlying interface, I believe that using get_wanx_ifname should do the trick.

Note: I am not sure if the same check should be added for internet over an USB connection (few lines below), as I am unable to test this.