sfc: Reduce the size of struct efx_tx_buffer

Remove unmap_addr since it can be calculated from dma_addr, len and
unmap_len.  This saves 4-16 bytes.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>