batman-adv: bla: fix type misuse for backbone_gw hash indexing

It seems that due to a copy & paste error the void pointer
in batadv_choose_backbone_gw() is cast to the wrong type.

Fixing this by using "struct batadv_bla_backbone_gw" instead of "struct
batadv_bla_claim" which better matches the caller's side.

For now it seems that we were lucky because the two structs both have
their orig/vid and addr/vid in the beginning. However I stumbled over
this issue when I was trying to add some debug variables in front of
"orig" in batadv_backbone_gw, which caused hash lookups to fail.

Fixes: 07568d03 ("batman-adv: don't rely on positions in struct for hashing")
Signed-off-by: Linus Lüssing <ll@simonwunderlich.de>
Signed-off-by: Sven Eckelmann <sven@narfation.org>