mv643xx_eth: get rid of individual port config extend register bit defines

The port config extend register is never changed at run time.
Document the meaning of the initial value, and delete the defines
for the individual bits in this register.

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