bgmac: simplify/optimize rx DMA error handling

Allocate a new buffer before processing the completed one. If allocation
fails, reuse the old buffer.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Acked-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>