usb: roles: intel_xhci: Fix Unbalanced pm_runtime_enable

Add missing pm_runtime_disable() to remove(), in order to avoid
an Unbalanced pm_runtime_enable when the module is removed and
re-probed.

Error log:
root@intel-corei7-64:~# modprobe -r intel_xhci_usb_role_switch
root@intel-corei7-64:~# modprobe intel_xhci_usb_role_switch
intel_xhci_usb_sw intel_xhci_usb_sw: Unbalanced pm_runtime_enable!

Fixes: cb296846

 (usb: roles: intel_xhci: Enable runtime PM)
Cc: <stable@vger.kernel.org>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Wan Ahmad Zainie <wan.ahmad.zainie.wan.mohamad@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>