AP: Extend the BSS bridge neighbor entry management to support IPv6

AP: Extend the BSS bridge neighbor entry management to support IPv6

This allows adding/deleting an IPv6 neighbor entry to/from the bridge,
to which the BSS belongs. This commit adds the needed functionality in
driver_nl80211.c for the Linux bridge implementation. In theory, this
could be shared with multiple Linux driver interfaces, but for now, only
the main nl80211 interface is supported.

Signed-off-by: Kyeyoon Park <kyeyoonp@qca.qualcomm.com>