usb: xhci: Use IS_ENABLED() macro

Using the IS_ENABLED() macro can make the code shorter and easier to read.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>