ftgmac100: Split tx packet freeing from ftgmac100_tx_complete_packet()

This moves the packet freeing to a separate function
which is also used by ftgmac100_free_buffers() and will
be used more in the error path of fragmented sends.

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