igb: Loopback functionality supports for i350 devices

This patch adds VMDq loopback pf support for i350 devices. The patch
is necessary since the register that enabled loopback was moved and
renamed from DTXSWC to TXSWC.

Signed-off-by: "Akeem G. Abodunrin" <akeem.g.abodunrin@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>