kernel/modules: fix kmod-mdio-gpio module dependencies

On some targets this module depends on kmod-of-mdio.

This is similar to the fixes done in:
dc629d9cf5 ("kernel: fix kmod-switch-rtl8366-smi dependency")
56bd23cf52 ("kernel: let kmod-rtl8366-smi conditionally depend on kmod-of-mdio")

Fixes: 32f32398af ("kernel/modules: add kmod-mdio-gpio module")
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

package/kernel/linux/modules/netdevices.mk

@@ -113,7 +113,7 @@ $(eval $(call KernelPackage,mii))
 define KernelPackage/mdio-gpio
   SUBMENU:=$(NETWORK_DEVICES_MENU)
   TITLE:= Supports GPIO lib-based MDIO busses
-  DEPENDS:=+kmod-libphy @GPIO_SUPPORT
+  DEPENDS:=+kmod-libphy @GPIO_SUPPORT +(TARGET_armvirt||TARGET_brcm2708_bcm2708):kmod-of-mdio
   KCONFIG:= \
 	CONFIG_MDIO_BITBANG \
 	CONFIG_MDIO_GPIO