batman-adv: pass the WIFI flag from the local to global entry

in case of client roaming a new global entry is added while a corresponding
local one is still present. In this case the node can safely pass the WIFI flag
from the local to the global entry.

This change is required to let the AP-isolation correctly working in case of
roaming: if a generic WIFI client C roams from node A to B, A adds a global
entry for C without adding any WIFI flag. The latter will be set only later,
once A has received C's advertisement from B. In this time period the
AP-Isolation (if enabled) would not correctly work since C is not marked as
WIFI, so allowing it to communicate with other WIFI clients.

Signed-off-by: Antonio Quartulli <ordex@autistici.org>