mv643xx_eth: detect alternate TX BW control register location

Some SoCs have the TX bandwidth control registers in a slightly
different place.  This patch detects that case at run time, and
re-directs accesses to those registers to the proper place at
run time if needed.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Acked-by: Dale Farnsworth <dale@farnsworth.org>