gianfar: Add macros for stepping through BDs

gianfar: Add macros for stepping through BDs

This code is based strongly on code from Dai Haruki <Dai.Haruki@freescale.com>.

The gianfar Buffer Descriptors are arranged in a circular array, the end of
which is denoted by setting the "WRAP" bit in the descriptor.  However, the
software knows the end of the ring because it knows how many descriptors are
there.  Rather than check each descriptor for whether the WRAP bit is set,
use pointer math to determine where the next BD is.  This is also useful for
when we want to look at BDs other than the very next one (for Scatter-Gather).

Signed-off-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>