wifi: wilc1000: fix potential memory leak in wilc_mac_xmit()

The wilc_mac_xmit() returns NETDEV_TX_OK without freeing skb, add
dev_kfree_skb() to fix it. Compile tested only.

Fixes: c5c77ba1 ("staging: wilc1000: Add SDIO/SPI 802.11 driver")
Signed-off-by: Zhang Changzhong <zhangchangzhong@huawei.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/1668684964-48622-1-git-send-email-zhangchangzhong@huawei.com