qeth: add layer 2 RX/TX checksum offloading

Checksum offloading for send and receive is already
supported for layer 3 (IP layer). This patch
adds support for RX and TX hardware checksum offloading
for layer 2 (MAC layer). The hardware calculates the checksum
for IP UDP and TCP packets.

This patch moves the hardware checksum offloading setup
to the set of common functions in qeth_core_main.c.
Layer 2 and layer 3 now simply call the same common functions.

Also note that TX checksum offloading is always enabled.
The device driver relies on the TCP/IP stack to make use of
this feature.

Signed-off-by: Thomas Richter <tmricht@linux.vnet.ibm.com>
Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Reviewed-by: Eugene Crosser <Eugene.Crosser@ru.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>