Fix STM32 radio driver when bandwidth is 0

[LukaB]

Summary of changes

A bug was detected when the uint8_t SUBGRF_GetFskBandwidthRegValue( uint32_t bandwidth )
is called and the bandwidth argument has a value of 0.

Comparing the code to the STMCubeWL 1.1 we can see that an if statement is missing to
address the condition where bandwidth is equal to 0.

Added the if statement to the radio driver to account for this edge case to prevent entering while (1) loop.

Impact of changes

Migration actions required

Documentation

None required.
But to replicate, I was using AS923-1, and then set my Gateway to have a maximum allowed data rate of DR_7.
The mBed device connected and quickly maxed out to DR_7 at which point the above function as called with bandwidth == 0.

Code from radio_driver.c (Line 1143):

uint8_t SUBGRF_GetFskBandwidthRegValue( uint32_t bandwidth )
{
    uint8_t i;

    if( bandwidth == 0 )
    {
        return( 0x1F );
    }

    /* ST_WORKAROUND_BEGIN: Simplified loop */
    for( i = 0; i < ( sizeof( FskBandwidths ) / sizeof( FskBandwidth_t ) ); i++ )
    {
        if ( bandwidth < FskBandwidths[i].bandwidth )
        {
            return FskBandwidths[i].RegValue;
        }
    }
    /* ST_WORKAROUND_END */
    // ERROR: Value not found
    while( 1 );
}

---

Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

---

Test results

[] No Tests required for this change (E.g docs only update)
[X] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

---

Reviewers

@jeromecoutant @hallard

---

[jeromecoutant]

Ref: https://github.com/STMicroelectronics/STM32CubeWL/blob/main/Middlewares/Third_Party/SubGHz_Phy/stm32_radio_driver/radio_driver.c#L1147

@ludoch-stm

[ciarmcom]

@LukaB, thank you for your changes.
@jeromecoutant @ARMmbed/mbed-os-maintainers please review.

Pull request has been modified.

[mbed-ci]

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