staging: wfx: drop useless field from struct wfx_tx_priv

staging: wfx: drop useless field from struct wfx_tx_priv

The device need to receive a skb with necessary space for the ICV. So,
the driver adds this space before to send the frame.

Currently, once the frame is sent, the driver restore the original
content of the skb. However, this step is useless. Mac80211 don't do it
when software encryption is enabled.

Once we have removed this step, it appears that it is no more necessary
to keep hw_key in tx_priv. Then, it is possible to simplify a bunch of
code in the Tx path.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200825085828.399505-6-Jerome.Pouiller@silabs.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>