mv643xx_eth: delete unused port serial control register bit defines

Over half of the port serial control register bit defines are never
used, and the PORT_SERIAL_CONTROL_DEFAULT_VALUE define is never used
either.  Keep only those defines that are actually used.

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