e1000: clear ip csum info from context descriptor

Since the driver sets the IP checksum insertion bit (IXSM in Status
field) in transmit context descriptors, it should clear the IP checksum
bits of any garbage so as not to confuse the hardware.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>