request_queue.rpm_status is assigned values of the rpm_status enum only,
so reflect that in its type.

Note that including <linux/pm.h> is (currently) a no-op, as it is
already included through <linux/genhd.h> and <linux/device.h>, but it is
better to play it safe.
Signed-off-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Acked-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>