net: systemport: rewrite bcm_sysport_rx_refill

Currently, bcm_sysport_desc_rx() calls bcm_sysport_rx_refill() at the end of Rx
packet processing loop, after the current Rx packet has already been passed to
napi_gro_receive(). However, bcm_sysport_rx_refill() might fail to allocate a new
Rx skb, thus leaving a hole on the Rx queue where no valid Rx buffer exists.

To eliminate this situation:

1. Rewrite bcm_sysport_rx_refill() to retain the current Rx skb on the
Rx queue if a new replacement Rx skb can't be allocated and DMA-mapped.
In this case, the data on the current Rx skb is effectively dropped.

2. Modify bcm_sysport_desc_rx() to call bcm_sysport_rx_refill() at the
top of Rx packet processing loop, so that the new replacement Rx skb is
already in place before the current Rx skb is processed.

This is loosely inspired from d6707bec ("net: bcmgenet: rewrite
bcmgenet_rx_refill()")

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