usb: host: ehci-msm: Register usb shutdown function

Registering usb_hcd_platform_shutdown to be called during
shutdown. This is a generic function that performs the
generic host stack's shutdown. It ensures that USB
operations do not continue while kexec boots a new kernel.

Signed-off-by: Azriel Samson <asamson@codeaurora.org>
Signed-off-by: Timur Tabi <timur@codeaurora.org>
Reviewed-by: Andy Gross <andy.gross@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>