libertas: fix memory corruption in lbs_remove_card()

"priv" is stored at the end of the wiphy structure, which is freed
during the call to lbs_cfg_free(). It must not be touched afterwards.

Remove the unnecessary NULL assignment causing this memory corruption.

Signed-off-by: Daniel Drake <dsd@laptop.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>