Staging: rtl8188eu: os_dep: Remove NULL test before rtw_free_netdev

The function rtw_free_netdev test whether its argument is NULL and
return immediately. So NULL test before this function is not needed.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>