Fix UART RevB Driver not cleaning up Async Transactions

Currently, the private AsyncTxRequests and AsyncRxRequests arrays are used to track transactions in progress for uart_revb.c.
The Tx/Rx Async Callbacks check for a callback function before clearing the Requests:

```C
mxc_uart_req_t *req = (mxc_uart_req_t *)AsyncTxRequests[uart_num];
if ((req != NULL) && (req->callback != NULL)) {
    AsyncTxRequests[uart_num] = NULL;
    req->callback(req, retVal);
}
```

This means that if the user does not supply an explicit callback function, a transaction never gets cleared, causing problems after just one transaction.

This commit resolves this by moving the cleanup of this important struct outside of the conditional check for a callback function.

```C
mxc_uart_req_t *req = (mxc_uart_req_t *)AsyncTxRequests[uart_num];
if ((req != NULL) && (req->callback != NULL)) {
    req->callback(req, retVal);
}

// Cleanup Async Transaction
AsyncTxRequests[uart_num] = NULL;
```

Signed-off-by: Brandon Hurst <brandon.hurst@analog.com>

Libraries/PeriphDrivers/Source/UART/uart_revb.c

@@ -749,10 +749,12 @@ int MXC_UART_RevB_AsyncTxCallback(mxc_uart_revb_regs_t *uart, int retVal)
 
     mxc_uart_req_t *req = (mxc_uart_req_t *)AsyncTxRequests[uart_num];
     if ((req != NULL) && (req->callback != NULL)) {
-        AsyncTxRequests[uart_num] = NULL;
         req->callback(req, retVal);
     }
 
+    // Cleanup Async Transaction
+    AsyncTxRequests[uart_num] = NULL;
+
     return E_NO_ERROR;
 }
 
@@ -762,10 +764,12 @@ int MXC_UART_RevB_AsyncRxCallback(mxc_uart_revb_regs_t *uart, int retVal)
 
     mxc_uart_req_t *req = (mxc_uart_req_t *)AsyncRxRequests[uart_num];
     if ((req != NULL) && (req->callback != NULL)) {
-        AsyncRxRequests[uart_num] = NULL;
         req->callback(req, retVal);
     }
 
+    // Cleanup Async Transaction
+    AsyncRxRequests[uart_num] = NULL;
+
     return E_NO_ERROR;
 }