quoted

		...
		depends on MII if MII != n

except that Kconfig doesn't comprehend conditionals like that.

You can express this in Kconfig:
	depends MII || MII=n

Except that:

Warning! Found recursive dependency: USB_USBNET USB_NET_AX8817X MII USB_USBNET

I think this is another case where Kconfig gets in the way and forces
introduction of a pseudovariable.  I'll give that a try.

But my suggestion was:
#if defined(CONFIG_MII) || (defined(CONFIG_MII_MODULE) && defined(MODULE))

Or simply select MII ...

Nope; those both prevent completely legit configurations.
MII is not required, except for those two adapter options.