treewide: remove "+" sign for increment with macaddr_add

treewide: remove "+" sign for increment with macaddr_add

Many people appear to use an unneeded "+" prefix for the increment
when calculating a MAC address with macaddr_add. Since this is not
required and used inconsistently [*], just remove it.

[*] As a funny side-fact, copy-pasting has led to almost all
    hotplug.d files using the "+", while nearly all of the
    02_network files are not using it.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>