stmmac: rework get_hw_feature function

On next GMAC IP generation (4.xx), the way to get hw feature
is not the same than on previous 3.xx. As it is hardware
dependent, the way to get hw capabilities should be defined in dma ops of
each MAC IP. It will avoid also a huge computation of hw capabilities in
stmmac_main.

Signed-off-by: Alexandre TORGUE <alexandre.torgue@st.com>
Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>