bgmac: increase rx ring size from 511 to 512

Limiting it to 511 looks like a failed attempt at leaving one descriptor
empty to allow the hardware to stop processing a buffer that has not
been prepared yet. However, this doesn't work because this affects the
total ring size as well

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>