ath6kl: fix null skb dereference in ath6kl_rx()

smatch found that skb might be null in some cases in ath6kl_rx():

ath6kl/txrx.c +1252 ath6kl_rx(222) error: potential null derefence 'skb'.

This will happen when ath6kl is in AP mode and two clients send traffic
to each other.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>