Lora: Fix rx datarate

[kivaisan]

Description

rx_config() incorrectly returns a physical layer datarate value when an index to datarate table should be returned.

This PR is for mbed-os 5.8 branch. The same bug will be fixed in master with PR #6808.

Pull request type

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

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 1960
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6846/

Triggering tests

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

[mbed-ci]

Test : SUCCESS

Build number : 1775
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6846/1775

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1610
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6846/

[adbridge]

Manually cherry picked to 5.8.5