mv643xx_eth: general cleanup

General cleanup of the mv643xx_eth driver.  Mainly fixes coding
style / indentation issues, get rid of some useless 'volatile's,
kill some more superfluous comments, and such.

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