net: xilinx: axienet: Implement BQL

Implement byte queue limits to allow queueing disciplines to account for
packets enqueued in the ring buffers but not yet transmitted.

Signed-off-by: Sean Anderson <sean.anderson@linux.dev>
Link: https://patch.msgid.link/20250214211252.2615573-1-sean.anderson@linux.dev


Signed-off-by: Jakub Kicinski <kuba@kernel.org>