mac80211: add #if guards against 5.4 compile failures

mac80211: add #if guards against 5.4 compile failures

Both struct net_device_path_ctx and struct net_device_path
are not available in 5.4. This causes an build error on the
bcm63xx target.

|mac80211/driver-ops.h: In function 'drv_net_fill_forward_path':
|driver-ops.h:1502:57: error: passing argument 4 of
|'local->ops->net_fill_forward_path' from incompatible pointer type
| [-Werror=incompatible-pointer-types]
| 1502 |                          ctx, path);
|      |                          ^~~
|      |                            |
|      |                         struct net_device_path_ctx *

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>