mv643xx_eth: get rid of individual port config register bit defines

The mv643xx_eth driver only ever changes bit 0 of the port config
register at run time, the rest of the register bits are fixed (and
always zero).  Document the meaning of the chosen default value,
and get rid of all the defines for each of the individual bits.

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