igb: report unsupported ethtool settings in set_coalesce

There are many settings possible using ethtool -C/--coalesce, but not
all of them are supported in igb. Report failure when an unsupported
option is set.

Signed-off-by: Todd Fujinaka <todd.fujinaka@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

diff --cc drivers/net/ethernet/intel/igb/igb_ethtool.c
Simple merge