de4x5: Remove developer debug feature about set/clear promisc

IFF_PROMISC flag shouldn't be set or cleared by drivers, because
whether device be promisc mode is decided by how many upper layer
callers being referenced to it.
And the promisc changing feature of de4x5 ioctl is developer debug
feature, we can remove it now.

Signed-off-by: Wang Chen <wangchen@cn.fujitsu.com>
Acked-by: Grant Grundler <grundler@parisc-linux.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>