Implement TSO on RealTek RTL8168/8111 C or later controllers.

Implement TSO on RealTek RTL8168/8111 C or later controllers.
RealTek changed TX descriptor format for later controllers so these
controllers require MSS configuration in different location of TX
descriptor. TSO is enabled by default for controllers that use new
descriptor format.
For old controllers, TSO is still disabled by default due to broken
frames under certain conditions but users can enable it.
Special thanks to Hayes Wang at RealTek.

MFC after:	2 weeks