usb: xhci: avoid warning for !PM_SLEEP

usb: xhci: avoid warning for !PM_SLEEP

If we build a kernel with PM_SUSPEND set and no PM_SLEEP,
we get a build warning in the xhci-plat driver about unused
functions.

To fix this, use "#ifdef CONFIG_PM_SLEEP", like we do in most
other drivers nowadays.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Mathias Nyman <mathias.nyman@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 274f6afa298791df97fd37b7bc9d8327f5cf6ee9)
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>