PM / Runtime: Use deferred_resume flag in pm_request_resume

This patch (as1307) adds a small optimization to
__pm_request_resume().  If the device is currently being suspended,
there's no need to queue a work routine to resume it.  Setting the
deferred_resume flag will suffice.  (There's also a minor improvement
to the function's code layout: An unnecessary "else" is removed.)

Also, the patch clarifies the usage of the deferred_resume flag.  It
is meaningful only while a suspend is in progress, so it should be
cleared just before a suspend starts, not just after one ends.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>