stmmac: remove dead option in the driver's Kconfig

This patch removes the CPU_SUBTYPE_ST40 dependency in the
driver's Kconfig.
In fact, this option has been removed in the commit:
 f9669187
as reported by Christian Dietrich.

Note that the driver remains tested on STM platforms, only.

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