[change] Invert upload/download on interfaces that are member of a bridge

[openwisp]

[change] Invert upload/download on interfaces that are member of a bridge

[nemesifier]

Addendum: I'm trying to do some more research on the subject before proceeding with this: https://forum.openwrt.org/t/inverted-tx-rx-packets-in-bridged-interfaces/85093.

[nemesifier]

During testing I found out my assumption was wrong.
I think I understood now: is not that rx/tx are inverted, is that the interface tx/rx statistics of a wifi access point interface installed on a router have a different perspective: for the wifi interface, every packet that the user downloads is transmitted (tx) to the user, while every packet that the user uploads is received (rx) by the interface, that explains why tx bytes is greater than rx bytes on these interfaces.
I do not think it has to do anything with the fact that the interface is bridged: a tap (layer2 VPN) member of a bridge which is used to tunnel traffic still shows received packets greater than sent packets, so that's why I changed the code further in 266d98a to ensure the inversion is done only on wifi acess points and I made clear in the comment that we do this to show data from the perspective of the user rather than the perspective of the user.