stmmac: use mdelay on timeout of sw reset

This patch uses an mdelay to manage the timeout on
sw reset to be independant of cpu_clk.

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