USB: gadget: gadget zero cleanups

Clean up gadget zero, using newer APIs and conventions:

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

 - Remove many now-needless #includes

 - Use the VERBOSE_DEBUG convention

 - Some whitespace fixes.

 - A few comment updates
 
 - Plus a few other small cleanups:  don't pass gfp_t around when it's
   always going to be GFP_ATOMIC, and do static init of serial number.

Also go to straight GPL; there's no real point in dual licensing this
stuff any more.

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