stmmac: fix platform driver unregistering

This patch fixes platform device drivers unregistering and adds proper error
handing on module loading.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: netdev@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>