e1000e: Don't zero out buffer address in rx descriptor

The e1000e emulation zeroes out any used rx descriptor and then writes a
completely newly constructed value there. By doing this, it doesn't only
update the write-back area of the descriptors (as it's supposed to do),
but it also clears the buffer address, which real hardware doesn't do.

The spec explicitly mentions in chapter 7.1.8 that it is valid for a
driver to reuse a descriptor and only update the status field while
doing so, i.e. reusing the old buffer address:

    If software statically allocates buffers, and uses memory read to
    check for completed descriptors, it simply has to zero the status
    byte in the descriptor to make it ready for reuse by hardware.

This patch fixes the behaviour to leave the buffer address in
descriptors unchanged even after the descriptor has been used.

Signed-off-by: Kevin Wolf <mail@kevin-wolf.de>
Reviewed-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
