net: bcmgenet: do not set packet length for RX buffers

Hardware will provide this information as soon as we will start
processing incoming packets, so there is no need to set the RX buffer
length during buffer allocation.

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