stmmac: Fix for higher mtu size handling

stmmac: Fix for higher mtu size handling

For the higher mtu sizes requiring the buffer size greater than 8192,
the buffers are sent or received using multiple dma descriptors/ same
descriptor with option of multi buffer handling.
It was observed during tests that the driver was missing on data
packets during the normal ping operations if the data buffers being used
catered to jumbo frame handling.

The memory barrriers are added in between preparation of dma descriptors
in the jumbo frame handling path to ensure all instructions before
enabling the dma are complete.

Signed-off-by: Deepak Sikri <deepak.sikri@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>