USB: fix autosuspend bug in usb-serial

This patch (as1437) fixes a bug in the usb-serial autosuspend
handling.  Since the usb-serial core now has autosuspend support, it
must set the .supports_autosuspend member in every serial driver it
registers.  Otherwise the usb_autopm_get_interface() call won't work.

This fixes Bugzilla #23012.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
CC: stable@kernel.org
Reported-by: Kevin Smith <thirdwiggin@gmail.com>
Reported-and-tested-by: Simon Gerber <gesimu@gmail.com>
Reported-and-tested-by: Matteo Croce <matteo@openwrt.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>