LoRa: Stop processing MAC commands if command id is unknown

[kivaisan]

Description

The length of a MAC command is not explicitly given and must be
implicitly known by the MAC implementation. Therefore unknown MAC
commands cannot be skipped and the first unknown MAC command
terminates the processing of the MAC command sequence.

This commit fixes the bug where MAC command processing was not
terminated when unknown MAC command was received.

Pull request type

[X] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Breaking change

[kivaisan]

@hasnainvirk @AnttiKauppila please review.

[hasnainvirk]

@kivaisan Please check why the build failed.

[kivaisan]

Build failure does not seem to be related to this PR.

[hasnainvirk]

@0xc0170 @cmonr Please invoke CI here.

[cmonr]

Thanks @hasnainvirk. Travis jobs restarted. Silly network issues...

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 3113
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7935/

Triggering tests

/morph test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2720
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7935/

[mbed-ci]

Test : SUCCESS

Build number : 2905
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7935/2905