staging: nvec: release sync write lock in error case

We forgot to release the sync write lock in case the async write
fails. Found by rpiloose on IRC.
Signed-off-by: Marc Dietrich <marvin24@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/nvec/nvec.c

@@ -294,8 +294,10 @@ struct nvec_msg *nvec_write_sync(struct nvec_chip *nvec,
 
 	nvec->sync_write_pending = (data[1] << 8) + data[0];
 
-	if (nvec_write_async(nvec, data, size) < 0)
+	if (nvec_write_async(nvec, data, size) < 0) {
+		mutex_unlock(&nvec->sync_write_mutex);
 		return NULL;
+	}
 
 	dev_dbg(nvec->dev, "nvec_sync_write: 0x%04x\n",
 					nvec->sync_write_pending);