xhci: Fix compile with CONFIG_USB_SUSPEND=n

The USB 2.0 Link PM code is conditionally compiled when
CONFIG_USB_SUSPEND=y.  I believe that's a mistake, since Link PM is not
directly related to USB device suspend and Link PM is implemented
without relying on any of the suspend code in the USB core.  For now,
keep the USB 2.0 Link PM code conditionally compiled if
CONFIG_USB_SUSPEND=y.

This patch does move the code to implement USB 3.0 Link PM out of the
xHCI driver #ifdefs for CONFIG_USB_SUSPEND and moves it into a section
dependent on CONFIG_PM.  The USB core functions for USB 3.0 Link PM are
already conditionally compiled when CONFIG_PM=y.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>