ftgmac100: Store tx skbs in a separate array

Rather than in the descriptor. The descriptor is mapped non-cachable
and rather slow to access.

Since to do that we need to keep track of the tx "pointer" we also
have no use of all the accesors to manipulate it, just open code
it, it's as clear and will help when adding fragmented sends.

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