qeth: l3 send dhcp in non pass thru mode

dhcp frames are valid IPv4 packets so there is no need to send them
in pass thru mode. This allows dhcp packets to pass HiperSockets.
Also the dhcp release frame is send out correctly with this patch.

Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>