stmmac: display the descriptors if DES0 = 0

It makes sense to display the descriptors even if
DES0 is zero. This helps for example in case of it
is needed to dump rx write-back descriptors to get
timestamp status.

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