alx: fix alx_poll()

Commit d75b1ade ("net: less interrupt masking in NAPI") uncovered
wrong alx_poll() behavior.

A NAPI poll() handler is supposed to return exactly the budget when/if
napi_complete() has not been called.

It is also supposed to return number of frames that were received, so
that netdev_budget can have a meaning.

Also, in case of TX pressure, we still have to dequeue received
packets : alx_clean_rx_irq() has to be called even if
alx_clean_tx_irq(alx) returns false, otherwise device is half duplex.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Fixes: d75b1ade ("net: less interrupt masking in NAPI")
Reported-by: Oded Gabbay <oded.gabbay@amd.com>
Bisected-by: Oded Gabbay <oded.gabbay@amd.com>
Tested-by: Oded Gabbay <oded.gabbay@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>