igb: restore workaround errata with wthresh on 82576

igb: restore workaround errata with wthresh on 82576

The 82576 has known issues which require the write threshold to be set to 1.
See:
	http://download.intel.com/design/network/specupdt/82576_SPECUPDATE.pdf

If not then single packets will hang in transmit ring until more arrive.
Simple tests like ping will fail.

The workaround was in the wrong file (commit a30ebfbb8c3a).
Move it in igb one to restore original patch (7e9e49feea).

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>