fix(PeriphDrivers): Fix UART RevB Driver not cleaning up Async Transactions

[Brandon-Hurst]

Description

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:

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.

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;

Testing

This was tested as part of development for UART functionality in the CircuitPython BUSIO.UART module:
https://github.com/Brandon-Hurst/circuitpython/blob/ports/analog/add-busio/ports/analog/common-hal/busio/UART.c

I found that it was necessary to submit a callback function for the Async API to work correctly. This fix is to correct this, so the user doesn't have to supply a callback to have the Async Requests cleaned up.

Checklist Before Requesting Review

• PR Title follows correct guidelines.
• Description of changes and all other relevant information.
• (Optional) Link any related GitHub issues using a keyword
• (Optional) Provide info on any relevant functional testing/validation. For API changes or significant features, this is not optional.