ibmvnic: map L2/L3/L4 header descriptors to firmware

Allow the VNIC driver to provide descriptors containing
L2/L3/L4 headers to firmware.  This feature is needed
for greater hardware compatibility and enablement of checksum
and TCP offloading features.

A new function is included for the hypervisor call,
H_SEND_SUBCRQ_INDIRECT, allowing a DMA-mapped array of SCRQ
descriptor elements to be sent to the VNIC server.

These additions will help fully enable checksum offloading as
well as other features as they are included later.

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