net: stmmac: replace unsigned by u32

checkpatch complains about two unsigned without type after.
Since the value return is u32, it is simpler to replace it by u32 instead
of "unsigned int"

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>