LinuxKPI 802.11: change type of bssid in struct ieee80211_bss_conf

Enabling other driver code found that the bssid in
struct ieee80211_bss_conf is not an array but expected to be
a const pointer (const, != NULL checks).
Adjust accordingly in the header and in the LinuxKPI compat code.
There initialization now needs to be a static array always present
as we need a value before we will have a BSS (node in scan_to_auth)
as the mac80211 driver (*handlers) are expecting the pointer to be
not NULL (copying without checks).
This is a pre-req to enable d3 (CONFIG_PM[_SLEEP]) in the future.

Tested by: Tomoaki AOKI (junchoon dec.sakura.ne.jp)
Tested by: Berislav Purgar (bpurgar gmail.com)
Sponsored by: The FreeBSD Foundation
MFC after: 3 days