gianfar: Fix a bug where the pointer never moves for dma_unmap...

The loop that unmaps all of the TX Buffer Descriptors never actually
moves the txbd pointer, so we were just repeatedly unmapping the first one.

Signed-off-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>