USB: chaoskey: Use kasprintf() over strcpy()/strcat()

Instead of kmalloc() with manually calculated values followed by
multiple strcpy()/strcat() calls, just fold it all into a single
kasprintf() call.

Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>