net: mv643xx_eth: ensure coalesce settings survive read-modify-write
The coalesce settings behave badly when changing just one value:
... # ethtool -c eth0
rx-usecs: 249
... # ethtool -C eth0 tx-usecs 250
... # ethtool -c eth0
rx-usecs: 248
This occurs due to rounding errors when calculating the microseconds
value - the divisons round down. This causes (eg) the rx-usecs to
decrease by one every time the tx-usecs value is set as per the above.
Fix this by making the divison round-to-nearest.
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>