Intel xhci: Only switch the switchable ports

With a previous patch to enable the EHCI/XHCI port switching, it switches
all the available ports.

The assumption is not correct because the BIOS may expect some ports
not switchable by the OS.

There are two more registers that contains the information of the switchable
and non-switchable ports.

This patch adds the checking code for the two register so that only the
switchable ports are altered.

This patch should be backported to kernels as old as 3.0, that contain
commit ID 69e848c2 "Intel xhci: Support
EHCI/xHCI port switching."

Signed-off-by: Keng-Yu Lin <kengyu@canonical.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: stable@vger.kernel.org