stmmac: fix driver built w/ w/o both pci and platf modules

The commit ba27ec66 fixes the Kconfig of the
driver when built as module allowing to select/unselect
the PCI and Platform modules that are not anymore mutually
exclusive. This patch fixes and guarantees that the driver
builds on all the platforms w/ w/o PCI and when select/unselect
the two stmmac supports. In case of there are some problems
on both the configuration and the pci/pltf registration the
module_init will fail.

v2: set the CONFIG_STMMAC_PLATFORM enabled by default.
I've just noticed that this can actually help on
some configurations that don't enable any STMMAC
options by default (e.g. SPEAr).

v3: change printk level when do not register the driver.

Reported-by: Fengguang Wu <wfg@linux.intel.com>
Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>