net/igb/e1000/e1000e: more robust ethtool duplex/speed configuration

This makes sure that one cannot request a 99Mbps full-duplex and get a
100Mbps half-duplex configuration in return due to the way the
speed/duplex parameters are handled internally.

Tested: e1000 works
Signed-off-by: David Decotigny <decot@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>