COSA/SRP: convert channel_data.rsem to mutex

COSA/SRP driver: The semaphore channel_data.rsem is used as a mutex,
convert it to the mutex API

Signed-off-by: Matthias Kaehlcke <matthias@kaehlcke.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>