powerpc/usb: fix bug of kernel hang when initializing usb

If USB UTMI PHY is not enable, writing to portsc register will lead to
kernel hang during boot up.

Signed-off-by: Shengzhou Liu <Shengzhou.Liu@freescale.com>
Reported-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>