thunderbolt: test: Reinstate a few casts of bitfields

Partially revert "thunderbolt: test: Remove some casts which are no
longer required". It turns out that typeof() doesn't support bitfields,
so these still need to be cast to the appropriate enum.

The only mention of typeof() and bitfields I can find is in the proposal
to standardise them:
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2619.htm

This was caught by the kernel test robot:
https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org/thread/FDKBHAV7QNLNFU5NBI2RKV56DWDSOLGM/



Fixes: 8f0877c2 ("thunderbolt: test: Remove some casts which are no longer required")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>