Cellular: Fix greentea network set credentials test case

[mirelachirica]

Description

Fix set credentials test case to pass in case AT+CGAUTH is not supported by modem.

Pull request type

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

[NirSonnenschein]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 3273
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/8268/

Triggering tests

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

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2898
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/8268/

[mbed-ci]

Test : SUCCESS

Build number : 3081
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8268/3081