thunderbolt: test: Remove some casts which are no longer required

With some of the stricter type checking in KUnit's EXPECT macros
removed, several casts in the thunderbolt KUnit tests are no longer
required.

Remove the unnecessary casts, making the conditions clearer.

Signed-off-by: David Gow <davidgow@google.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>