stmmac: do not use strict_strtoul but kstrtoint

This patch replaces the obsolete strict_strtoul with kstrtoint.

v2: also removed casting on kstrtoul.
v3: use kstrtoint instead of kstrtoul due to all vars are integer.
    thanks to E. Dumazet.

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