usb: xhci: fix comment for PORT_DEV_REMOVE

According to xHCI specification, PORT_DEV_REMOVE(bit 30) in PORTSC
true means "Device is non-removable".

Reported-by: Juro Bystricky <jurobystricky@hotmail.com>
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>