thunderbolt: Remove superfluous check

The key size is tested by hex2bin() already (as '\0' isn't an hex digit)

Suggested-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Yehezkel Bernat <yehezkel.bernat@intel.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>