net: systemport: fix tx work done in TX napi poll

With commit d75b1ade ("net: less interrupt masking in NAPI") napi
repoll is done only when work_done == budget. bcm_sysport_tx_poll()
always returns 0 whether or not we completed the poll quantum.

Fix this by returning either 0 when we did complete the TX ring reclaim,
or budget to trigger a repoll.

Fixes: d75b1ade ("net: less interrupt masking in NAPI")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>