macb: allow GEM to have configurable receive buffer size

GEM has configurable receive buffer sizes so requires this to be
programmed up.  Any size < 2048 and a multiple of 64 bytes is permitted.

Signed-off-by: Jamie Iles <jamie@jamieiles.com>
Acked-by: David S. Miller <davem@davemloft.net>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Tested-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>