USB: Prepare serial core for autosuspend.

Claim the interface for a USB to serial converter when the tty is open,
and release the interface when the tty is closed.

If a driver doesn't provide a resume function, use the generic resume
instead.

Make sure the generic resume function does not submit the URBs if we're
coming back from autosuspend.  On autoresume, we know that the open
function will be called next, which will attempt to submit the URBs.  If
we submit them in the resume function, the open will fail.

This works for:
 - autosuspend
 - suspending with the tty open or closed
 - hibernate with the tty closed

A hibernate (or a suspend that causes the USB subsystem to lose power)
has issues.  If you have the tty open when you hibernate, a new tty will
be created when the device re-enumerates during resume.

Signed-off-by: Sarah Sharp <sarah.a.sharp@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

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