net: broadcom: bcm4908_enet: fix RX path possible mem leak

After filling RX ring slot with new skb it's required to free old skb.
Immediately on error or later in the net subsystem.

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/20210224151842.2419-1-zajec5@gmail.com


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