Fix compilation of kernels with usb and fdt enabled, but no miibus

[stevek]

Summary

usb_ethernet.h includes a number of mii headers, but only does so in
order to have struct mii_data available. However, it only really needs
a forward declaration of struct mii_data for use in pointer form for
the return type of a function prototype.

Custom kernel configuration that have usb and fdt enabled, but no miibus,
end up with compilation failures because miibus_if.h will not get
generated.

Due to the above, the following changes have been made to usb_ethernet.h:

• remove the inclusion of mii headers
• forward-declare struct mii_data
• include net/ifq.h to satify the need for complete struct ifqueue

Test Plan

Built "kernels" successfully

[imp]

looks fine as far as it goes, but what's including this? usb_ethernet.c certainly needs miibus too compile...

[imp]

Ah, I see where it's used now. There are several alternatives, but this is the least bad one that comes to mind.
We have a little bit of dead code vs having #ifdefs for FDT in usb_ethernet.c.
It's more modular to have it like this, so I'm cool with it.