ath6kl: use netdev_features_t

Commit c8f44affb7 ("net: introduce and use netdev_features_t for
device features sets") added netdev_features_t to ndo_set_features.
Change ath6kl to use the new type.

This fixes a warning:

ath6kl/main.c:1170: warning: initialization from incompatible pointer type

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>