eth: fbnic: support devmem Tx

eth: fbnic: support devmem Tx

Support devmem Tx. We already use skb_frag_dma_map(), we just need
to make sure we don't try to unmap the frags. Check if frag is
unreadable and mark the ring entry.

  # ./tools/testing/selftests/drivers/net/hw/devmem.py
  TAP version 13
  1..3
  ok 1 devmem.check_rx
  ok 2 devmem.check_tx
  ok 3 devmem.check_tx_chunks
  # Totals: pass:3 fail:0 xfail:0 xpass:0 skip:0 error:0

Acked-by: Mina Almasry <almasrymina@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Link: https://patch.msgid.link/20250916145401.1464550-1-kuba@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>