macb: support statistics for GEM devices

GEM devices have a different number of statistics registers and they
are at a different offset to MACB devices.  Make the statistics
collection method dependent on device type.

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>