USB: gadget: serial gadget cleanups

Clean up the serial gadget, using newer APIs and conventions:

 - gadget_is_dualspeed() and gadget_is_otg() ... #ifdef removal

 - Remove many now-needless #includes

 - Use the DEBUG and VERBOSE_DEBUG conventions; turned up a bug in
   the original debug messaging

 - Various whitespace fixes.

This gave only a minor object code shrinkage, but the source looks
much cleaner in various places.

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