ibmvnic: Introduce xmit_more support using batched subCRQ hcalls

Include support for the xmit_more feature utilizing the
H_SEND_SUB_CRQ_INDIRECT hypervisor call which allows the sending
of multiple subordinate Command Response Queue descriptors in one
hypervisor call via a DMA-mapped buffer. This update reduces hypervisor
calls and thus hypervisor call overhead per TX descriptor.

Signed-off-by: Thomas Falcon <tlfalcon@linux.ibm.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>