device should not prevent the sleep from succeeding:

Failure to suspend a device won't matter, because the device will
automatically go into suspend mode when the USB bus stops carrying
packets.  (This might be less true for USB-3.0 devices, but let's not
worry about them now.)

Failure of a driver to suspend might lead to trouble later on when the
system wakes up, but it isn't sufficient reason to prevent the system
from going to sleep.

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