stmmac: fix csr clock divisor for 300MHz

This patch is to fix the csr clock in case of 300MHz is provided.

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