Nanostack Socket event handler fix

[juhhei01]

SOCKET_TX_DONE not generate event signal anymore for SOCKET_MODE_DATAGRAM.
Event handler try to read socket which is not necessary in that case.

Summary of changes

SOCKET_MODE_DATAGRAM nano stack SOCKET_TX_DONE can be suppress.

Impact of changes

None.

Migration actions required

None.

Documentation

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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

@teetak01 @kjbracey-arm @mikter @JarkkoPaso

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[juhhei01]

@kjbracey-arm @SeppoTakalo @mikter @JarkkoPaso Please review.

[kjbracey]

Maybe also remove the debug trace for the datagram case? Probably no point in logging something we're choosing to ignore.

What do others think?

EDIT: Although I note we are printing log on every single event code, including others we ignore, so doing so for this is at least consistent. If trimming this, probably should trim others.

[mbed-ci]

Test run: SUCCESS

Summary: 12 of 12 test jobs passed
Build number : 1
Build artifacts