net: stmmac: move hardware setup for stmmac_open to new function

This patch moves hardware setup part of the code in stmmac_open to a new
function stmmac_hw_setup, the reason for doing this is to make hw
initialization independent function so that PM functions can re-use it to
re-initialize the IP after returning from low power state.
This will also avoid code duplication across stmmac_resume/restore and
stmmac_open.

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>