ftgmac100: Don't clear tx desc fields unnecessarily

Those are non-cachable stores, let's avoid those we don't need. Remove
the helper, it's not particularly helpful and since it uses "priv"
I can't move it to the header file.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David S. Miller <davem@davemloft.net>