staging: unisys: include: Add comment next to mutex.

Checkpatch reports an error that no comment was next to the mutex lock.
Add an appropriate message for the lock.

Signed-off-by: David Kershner <david.kershner@unisys.com>
Reviewed-by: Tim Sell <timothy.sell@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>