USB: EHCI: prepare to make ehci-hcd a library module

This patch (as1624) prepares ehci-hcd for being split up into a core
library and separate platform driver modules.  A generic
ehci_hc_driver structure is created, containing all the "standard"
values, and a new mechanism is added whereby a driver module can
specify a set of overrides to those values.  In addition the
ehci_setup(), ehci_suspend(), and ehci_resume() routines need to be
EXPORTed for use by the drivers.

As a side effect of this change, a few routines no longer need to be
marked __maybe_unused.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
CC: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>