stmmac: do not call fix_mac_speed if NULL

On some platforms, fix_mac_speed is used for
configuring some sysconf registers according
to the working speed.
This patch fixes the fix_mac_speed invocation
that cannot be done if it is a NULL pointer.

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