FT: Use BSSID as r1_key_holder if no value is configured

FT: Use BSSID as r1_key_holder if no value is configured

r1_key_holder is an identifier that was always set to zero if unless
configured before.

See 11.6.1.7.4 of IEEE Std 802.11-2012 which reads
 "R1KH-ID is a MAC address of the holder of the PMK-R1 in the
  Authenticator of the AP"
See 12.2.2 of IEEE Std 802.11-2012 which reads
 "Each R0KH-ID and R1KH-ID is assumed to be expressed as a unique
  identifier within the mobility domain."
 "The R1KH-ID shall be set to a MAC address of the physical entity
  that stores the PMK-R1 ..."

Defaulting this to BSSID is a more reasonable value since we have not
rejected the missing r1_key_holder as invalid configuration.

Signed-off-by: Michael Braun <michael-dev@fami-braun.de>