stmmac: rework the routines to show the ring status

To avoid lot of check in stmmac_main for display ring management
and support the GMAC4 chip, the display_ring function is moved
into dedicated descriptor file.

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>