net: systemport: fix TX NAPI work done return value

Although we do not limit the number of packets the TX completion
function bcm_sysport_tx_reclaim() is allowed to reclaim, we were still
using its return value as-is. This means that we could hit the WARN() in
net/core/dev.c where work_done >= budget.

Make sure we do exit the NAPI context when the TX ring is empty, and
pretend there was no work to do.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>