usb: host: ehci: allow ehci_bus_resume symbol to be unused

All the platforms may not use ehci_bus_resume symbol in their
hc_driver structure. This may result in below warning:

drivers/usb/host/ehci-hub.c: warning: 'ehci_bus_resume' \
    defined but not used

In order to remove this warning add '__maybe_unused' annotation
to ehci_bus_resume function.

Change-Id: I3c97eed7555688d99d68618f3810197e574499cb
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>