eth: bnxt: add netmem TX support

eth: bnxt: add netmem TX support

Use netmem_dma_*() helpers and declare netmem_tx to support netmem TX.
By this change, all bnxt devices will support the netmem TX.

Unreadable skbs are not going to be handled by the TX push logic.
So, it checks whether a skb is readable or not before the TX push logic.

netmem TX can be tested with ncdevmem.c

Acked-by: Mina Almasry <almasrymina@google.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Reviewed-by: Michael Chan <michael.chan@broadcom.com>
Link: https://patch.msgid.link/20250619144058.147051-1-ap420073@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>