e1000: fix descriptor done flag with odd address

e1000: fix descriptor done flag with odd address

Header buffer address for header split will be filled with the physical
address for DMA, which is actually not needed at all, as header split
hasn't been supported. Hardware requires the least bit of header address
which is 'Descriptor Done' bit when write back should be set to 0 by driver.
The issue is that if the user wants to reserve an odd number of bytes between
the mbuf header and data buffer, the physical address to be filled in the
descriptor would happen to be odd. That means the DD bit would be set to
non-zero by driver. That will result in reporting descriptor done wrongly.

Signed-off-by: Wenzhuo Lu <wenzhuo.lu@intel.com>