wil6210: fix free'd memory access in wil_if_free()

wil6210: fix free'd memory access in wil_if_free()

In the wil_priv_deinit(), wdev->netdev is accessed, so free_netdev()
should not be called before mentioned call.

Set wdev->netdev to NULL Make sure no more attempts to use it.
It is used for debug printk if not NULL.

This fix kernel panic on module unload and in case error on probe;
if memory allocation debugging enabled.

Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>