ixgbe: fix ethtool register dump

1) reading some of the registers in our hardware causes them to clear,
so don't read ICR in the ethtool register dump function.
2) several register iterators were not iterating

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>