mv643xx_eth: rework interrupt coalescing, and export via ethtool

This patch:
- increases the precision of the receive/transmit interrupt
  coalescing register value computations by using 64bit temporaries;
- adds functions to read the current hardware coalescing register
  values and convert them back to usecs;
- exports the {get,set} {rx,tx} coal methods via the standard
  ethtool coalescing interface.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>