batman-adv: clarify CFG80211 dependency

The driver calls cfg80211_get_station, which may be part of a
module, so we must not enable BATMAN_ADV_BATMAN_V if
BATMAN_ADV=y and CFG80211=m:

net/built-in.o: In function `batadv_v_elp_get_throughput':
(text+0x5c62c): undefined reference to `cfg80211_get_station'

This clarifies the dependency to cover all combinations.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: c833484e ("batman-adv: ELP - compute the metric based on the estimated throughput")
Acked-by: Antonio Quartulli <a@unstable.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>