mv643xx_eth: move MIB offset defines into their only user

The only user of the ETH_MIB_VERY_LONG_NAME_HERE defines is the
eth_update_mib_counters() function.  Get rid of the defines by
open-coding the register offsets in the latter.

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