ath6kl: Fix tx packet drop in AP mode with bridge

skb is dropped in ath6kl_data_tx() when the headroom in skb
is insufficient. We hit this condition for every skb in AP mode
which is used with bridge, so all tx packets are getting dropped
when tried to send traffic to wireless client from bridge. Fix
this by reallocating the headroom instead of dropping the skb
when it has lesser headroom than needed.

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