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

Reviewed by: ian
Obtained from: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D21293