USB: Mark ehci_adjust_port_wakeup_flags as __maybe_unused

Mark ehci_adjust_port_wakeup_flags as __maybe_unused to avoid the following
warning when building the ehci-mxc driver:

  CC      drivers/usb/host/ehci-hcd.o
drivers/usb/host/ehci-hub.c:130: warning: 'ehci_adjust_port_wakeup_flags' defined but not used

Current ehci-mxc driver implementation does not support suspend/resume.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>