xhci: plat: Fix xhci_plat shutdown hung

xhci-plat Shutdown callback should check HCD_FLAG_HW_ACCESSIBLE
before accessing any register. This should avoid hung with access
controllers which support runtime suspend

Change-Id: I53eedf5b8f77dfdfa31d21b303da7058c5a86ba9
Signed-off-by: Lei wang <leiwan@codeaurora.org>