usb gadget: switch to put_char returning int

Signed-off-by: Alan Cox <alan@redhat.com>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>