net: stmmac: move dma allocation to new function

This patch moves dma resource allocation to a new function
alloc_dma_desc_resources, the reason for moving this to a new function
is to keep the memory allocations in a separate function. One more reason
it to get suspend and hibernation cases working without releasing and
allocating these resources during suspend-resume and freeze-restore
cases.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@st.com>
Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>