usb serial gadget: CDC ACM fixes

Based on a patch from <Aurel.Thomi@ruag.com>, this makes the
CDC-ACM support in the serial gadget handle the SET_LINE_CODING
and SET_CONTROL_LINE_STATE requests ... which should improve
interop with at least MS-Windows "usbser.sys" if not some other
ACM host drivers.

It also adds a few REVISIT comments where this code plays a bit
loose with the CDC ACM spec.  If this were used to hook up to a
real RS232 or modem link, those places would need a bit of work.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>