net/stmmac: mark probe function as __devinit

Driver probe functions are generally __devinit so they will be
discarded after initialization for non-hotplug kernels.
This was found by a new warning after patch 6a228452

 "stmmac: Add
device-tree support" adds a new __devinit function that is called
from stmmac_pltfr_probe.

Without this patch, building socfpga_defconfig results in:

WARNING: drivers/net/ethernet/stmicro/stmmac/stmmac.o(.text+0x5d4c): Section mismatch in reference from the function stmmac_pltfr_probe() to the function .devinit.text:stmmac_probe_config_dt()
The function stmmac_pltfr_probe() references
the function __devinit stmmac_probe_config_dt().
This is often because stmmac_pltfr_probe lacks a __devinit
annotation or the annotation of stmmac_probe_config_dt is wrong.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Stefan Roese <sr@denx.de>
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: netdev@vger.kernel.org
Acked-by: Stefan Roese <sr@denx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>