Here's an update for the Ethernet gadget that corresponds
to the earlier one for Gadget Zero ... it gets rid of almost
all the remaining controller-specific #ifdefs in this driver.
(And also lets the driver initialize using "dummy_hcd".)

This is a significant step towards doing hardware-specific
configuration at run time (or at least init-time) rather
than compile time, but other patches will be needed to take
it the rest of the way there.   (Especially considering the
RNDIS support...)  The runtime footprint of the driver shrank
a bit, mostly because things moved into the init section.