Lora: fix AU915 build

[kivaisan]

Description

There was a typo in the code making it fail to compile.

This PR should go to master. mbed-os 5.8.x branch is handled with a different PR (#6829) as code is moved to another place.

Pull request type

[X] Fix
[ ] Refactor
[ ] New target
[ ] Feature
[ ] Breaking change

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 1944
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6839/

Triggering tests

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

[mbed-ci]

Exporter Build : FAILURE

Build number : 1594
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6839/

[cmonr]

/morph export-build

[mbed-ci]

Exporter Build : FAILURE

Build number : 1600
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6839/

[cmonr]

/morph export-build

[mbed-ci]

Exporter Build : FAILURE

Build number : 1618
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6839/

[cmonr]

/morph export-build

[cmonr]

/morph test

[mbed-ci]

Test : FAILURE

Build number : 1786
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6839/1786

[kivaisan]

@cmonr this test error is not related to this PR?
[Error] main.cpp@17,0: #35: #error directive: [NOT_SUPPORTED] Tickless mode not supported for this target.

[mbed-ci]

Exporter Build : FAILURE

Build number : 1626
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6839/

[cmonr]

/morph export-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1641
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6839/

[0xc0170]

Failure looks like in the testing tools (CI issue), will report this (update: reported as an issue, will be checked)

/morph test

[mbed-ci]

Test : SUCCESS

Build number : 1804
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6839/1804

[kivaisan]

@cmonr This PR should not have 5.8.5 tag as target of this PR is 5.9.
This will be fixed to 5.8.x branch with own PR.