usb: xhci-plat: Prevent NULL access in runtime PM

During cases where runtime PM is invoked before xhci_plat_probe
has not finished initializing, the xhci_hcd member may not be
available yet. Guard against NULL pointer access by no-op'ing
the runtime_suspend/resume functions.

Change-Id: I759196ee187900c57497af7b098675b254fe23f8
Signed-off-by: Jack Pham <jackp@codeaurora.org>