PM: fix new mutex-locking bug in the PM core

This patch (as1041) fixes a bug introduced by the
acquire-all-device-semaphores reversion.  The error pathway of
dpm_suspend() fails to reacquire a mutex it should be holding.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>