stmmac: fix driver Kconfig when built as module

This patches fixes the driver when built as dynamic module.
In fact, the platform part cannot be built and the probe fails
(thanks to Bob Liu that reported this bug).

v2: as D. Miller suggested, it is not necessary to make the
pci and the platform code mutually exclusive.
Having both could also help, at built time ,to verify that
all the code is validated and compiles fine.

v3: removed wrong Reviewed-by from the patch

Reported-by: Bob Liu <lliubbo@gmail.com>
cc: Rayagond Kokatanur <rayagond@vayavyalabs.com>
Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>