cam-service: Reset semaphore count before destroying

The current error path of dispatch_thread is destroying the
semaphore before exit, but the semaphore can't be destroyed
until its count is zero, so the sem_destroy would return
an error and set errno to EBUSY, which will print an error
message.

To avoid this issue, before destroying the semaphore, take
all its count.

Add word to the .dictionary to please the spellchecker.

Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>