igb: fix LRO warning

This fix is only needed for 2.6.29.y tree, since in 2.6.30 and later IGB
has moved to using GRO instead of LRO.

igb supports LRO, but was not setting any hooks to the ->set_flags
ethtool_ops function.  This would trigger warnings if the user tried
to enable or disable LRO.

Based on the patch provided by Stephen Hemminger <shemminger@vyatta.com>

Reported-by: Sergey Kononenko <sergk@sergk.org.ua>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
CC: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>