stmmac: fix dma api misuse

Enabling DMA_API_DEBUG, warnings are reported at runtime
because the device driver frees DMA memory with wrong functions
and it does not call dma_mapping_error after mapping dma memory.

The first problem is fixed by of introducing a flag that helps us
keeping track which mapping technique was used, so that we can use
the right API for unmap.
This approach was inspired by the e1000 driver, which uses a similar
technique.

Signed-off-by: Andre Draszik <andre.draszik@st.com>
Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Reviewed-by: Denis Kirjanov <kda@linux-powerpc.org>
Cc: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>