STM32WL LoRaWAN set downlink preamble length to 8

[hallard]

Summary of changes

Set downlink preamble length to 8 instead of 5 (like uplink), this fix downlinks issues on US915 (and may be other)

Impact of changes

Migration actions required

Documentation

See #14449

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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)

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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

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Reviewers

@jeromecoutant

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[jeromecoutant]

code review, not verified

[0xc0170]

CI started

[mbed-ci]

Jenkins CI Test : ✔️ SUCCESS

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[hallard]

@0xc0170 we need to deploy some devices, could be awesome for people in charge of build if this PR could be merged. Any idea when it can be done?
Thanks.