serial: pch_uart: remove reference to .set_wake()

This callback is gone and not coming back, so will not be
supported later.

Cc: Johan Hovold <jhovold@gmail.com>
Acked-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>