usb: host: ehci: allow ehci_* symbols to be unused

not all platforms will use all of those ehci_*
symbols on their hc_driver structure. Sometimes
we might need to provide a modified version of
a certain method or not provide it at all, as is
the case with OMAPs which don't support port handoff
feature.

Whenever we compile a kernel for an OMAP board with
EHCI enabled, we get compile warnings:

drivers/usb/host/ehci-hub.c:1079: warning: 'ehci_relinquish_port' \
	defined but not used
drivers/usb/host/ehci-hub.c:1088: warning: 'ehci_port_handed_over' \
	defined but not used

In order to cleanup those warnings, we're adding
__maybe_unused annotation to those functions.

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