USB: serial gadget: descriptor cleanup

Bugfix some serial gadget descriptors:

 - Stop mangling the low bits (controller type ID) of bcdDevice;
   just use the high bits for a driver revision code.

 - Serial numbers that aren't specific to individual devices
   are useless; stop reporting "0" for this.

 - Since it's not part of a CDC-conformant function, the "bulk only"
   configuration shouldn't be using "CDC Data" as its interface class.
   Switch over to using CLASS_VENDOR_SPEC (different value, 0xff).

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: Al Borchers <alborchers@steinerpoint.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>