PM: allow for usage_count > 0 in pm_runtime_get()

This patch (as1308c) fixes __pm_runtime_get().  Currently the routine
will resume a device if the prior usage count was 0.  But this isn't
right; thanks to pm_runtime_get_noresume() the usage count can be
positive even while the device is suspended.

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