usb: xhci: make several functions static

Several functions have a single user in the same file where it
is defined. There's no need to expose it anywhere else.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>