qtnfmac: cleanup wdev structure between its uses

Driver uses statically allocated wdev structures for each virtual
interface. However wdev structure is not properly cleaned up between
its uses. As a result, various bugs appear when userspace tools
like hostapd were not gracefully stopped.

In particular, this commit fixes the following issue:
- start hostapd with more than 2 mBSS
- kill hostapd using SIGKILL
- start again hostapd with more than 2 mBSS
However only two mBSS entities will be started: primary
and the last BSS listed in hostapd config.

Signed-off-by: Sergey Matyukevich <sergey.matyukevich.os@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>