ibmvnic: Enable TSO support

This patch enables TSO support. It includes additional
buffers reserved exclusively for large packets. Throughput
is greatly increased with TSO enabled, from about 1 Gb/s to
9 Gb/s on our test systems.

Signed-off-by: Thomas Falcon <tlfalcon@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>