STM32: fix wrong LSE config in serial_baud function

[bcostm]

Description

The LSE configuration was incorrectly done in serial_baud function.

Fixes #8110

Pull request type

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

[adbridge]

Might need to get this into 5.10.1 as it could be related to one of the client issues. (IOTHAL-356)

[0xc0170]

Is this fixes the error: Cannot initialize RTC with LSE ?

[cmonr]

@adbridge It's unknown if it's able to correct the IOTHAL issue, since they're no longer able to reproduce the issue.

Will bring it in asap anyways, since RTC issues tend to be persistent across reflashes and can cause havoc on CI.

/morph build

FYI @ARMmbed/mbed-os-test

[mbed-ci]

Build : SUCCESS

Build number : 3117
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/8187/

Triggering tests

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

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2726
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/8187/

[mbed-ci]

Test : FAILURE

Build number : 2913
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8187/2913

[0xc0170]

/morph test

[mbed-ci]

Test : SUCCESS

Build number : 2916
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8187/2916