net: broadcom: bcm4908_enet: fix NAPI poll returned value

Missing increment was resulting in poll function always returning 0
instead of amount of processed packets.

Fixes: 4feffead ("net: broadcom: bcm4908enet: add BCM4908 controller driver")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20210224151842.2419-2-zajec5@gmail.com


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