usb: ehci-hcd: Fix ehci shutdown unclocked register access

ehci shutdown callback can access registers with the clocks
turned off if the USB controller is in low power mode. Check
HCD_FLAG_HW_ACCESSIBLE flag before accessing any register.

Change-Id: Iafc9a5057aa196131b3aa7a2f659b9f8f052ec7f
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>