fix(kernel-network-modules): also install modules from mdio subdirectory

A recent change in Linux kernel v5.10 split the MDIO drivers from the PHY
subdirectory into a new MDIO subdirectory [0]. This caused MDIO drivers
to not be included in the initramfs.

Also include the modules of this new subdirectory to prevent this issue.

[0]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=a9770eac511

Suggested-by: Peter Robinson <pbrobinson@gmail.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>

modules.d/90kernel-network-modules/module-setup.sh

@@ -37,6 +37,7 @@ installkernel() {
     dracut_instmods -o -P ".*${_unwanted_drivers}.*" -s "$_net_symbols" "=drivers/net"
     #instmods() will take care of hostonly
     instmods \
+        =drivers/net/mdio \
         =drivers/net/phy \
         =drivers/net/team \
         =drivers/net/ethernet \