igb: add support for seperate tx-usecs setting in ethtool

This patch adds support for a seperate tx-usecs interrupt moderation setting
in ethtool which is supported when tx and rx interrupt vectors are sperated.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>