USB: usb_debug: use the generic kfifo-based write implementation

Use the generic kfifo-based write implementation rather than allowing up
to 4000 8 byte urbs in the host stack queues.

Cc: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>