sfc: Extend struct efx_tx_buffer to allow pushing option descriptors

The TX path firmware for EF10 supports 'option descriptors' to control
offloads and various other features.  Add a flag and field for these
in struct efx_tx_buffer, and don't treat them as DMA descriptors on
completion.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>

diff --cc drivers/net/ethernet/sfc/farch.c
Simple merge

diff --cc drivers/net/ethernet/sfc/net_driver.h
Simple merge

diff --cc drivers/net/ethernet/sfc/tx.c
Simple merge