axienet_free_tx_chain returns the number of DMA descriptors it's
handled. However, axienet_tx_poll treats the return as the number of
packets. When scatter-gather SKBs are enabled, a single packet may use
multiple DMA descriptors, which causes incorrect packet counts. Fix this
by explicitly keepting track of the number of packets processed as
separate from the DMA descriptors.

Budget does not affect the number of Tx completions we can process for
NAPI, so we use the ring size as the limit instead of budget. As we no
longer return the number of descriptors processed to axienet_tx_poll, we
now update tx_bd_ci in axienet_free_tx_chain.

Fixes: 8a3b7a25 ("drivers/net/ethernet/xilinx: added Xilinx AXI Ethernet driver")
Signed-off-by: Sean Anderson <sean.anderson@linux.dev>
Link: https://patch.msgid.link/20240913145156.2283067-1-sean.anderson@linux.devSigned-off-by: Paolo Abeni <pabeni@redhat.com>