r8169: check that Realtek PHY driver module is loaded

Some users complained about problems with r8169 and it turned out that
the generic PHY driver was used instead instead of the dedicated one.
In all cases reason was that r8169.ko was in initramfs, but realtek.ko
not. Manually adding realtek.ko to initramfs fixed the issues.
Root cause seems to be that tools like dracut and genkernel don't
consider softdeps. Add a check for loaded Realtek PHY driver module
and provide the user with a hint if it's not loaded.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>