staging/wlan-ng: Fix 'Branch condition evaluates to a garbage value' in p80211netdev.c

clang/scan-build complains that:
p80211netdev.c:451:6: warning: Branch condition evaluates to a garbage
value
        if ((p80211_wep.data) && (p80211_wep.data != skb->data))
	            ^~~~~~~~~~~~~~~~~

This can happen in p80211knetdev_hard_start_xmit if
- if (wlandev->state != WLAN_DEVICE_OPEN) evaluates to true.
the execution flow then continues at the 'failed' label where
p80211_wep.data is used without being initialized first.

-> Initialize the data field to NULL to fix this issue.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>