drivers: can: mcan: tidy TX complete semaphore usage

"callback != NULL" is used to determine if the callback is in use.
The TX complete semaphone should only be given back after setting the
callback to NULL.

This would likely only be a race condition if the ISR is processed on a
different core to the TX call.

Signed-off-by: Grant Ramsay <gramsay@enphaseenergy.com>
(cherry picked from commit 12d6e268)