batman-adv: fix compilation bug in batadv_is_cfg80211_netdev()

batman-adv: fix compilation bug in batadv_is_cfg80211_netdev()

Because batman-adv is built under backports, not a clean linux tree,
the CONFIG_CFG80211 does not exist. The evaluation of IS_ENABLED() in
batadv_is_cfg80211_netdev() will be false, causing the funtion to
always return false.

This means that the wifi_flags of an interface don't get set, causing
batadv_is_wifi_hardif() to always return false. As a result,
batadv_v_elp_get_throughput() never tries to get the station info from
cfg80211, resulting in the following warning:

    batman_adv: bat0: WiFi driver or ethtool info does not provide
    information about link speeds on interface phy1-mesh0,
    therefore defaulting to hardcoded throughput values of 1.0 Mbps.

So replace CONFIG_CFG80211 with CPTCFG_CFG80211, which is the correct
macro to use under backports.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>