thunderbolt: Allow passing NULL to tb_ctl_free()

Following the usual pattern used in many places, we allow passing NULL
pointer to tb_ctl_free(). Then the user can call the function regardless
if it has allocated control channel or not making the code bit simpler.

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Andreas Noever <andreas.noever@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>