STM32: remove uart force_reset at init

[bcostm]

Description

Remove the force_reset during serial_init. This was causing a temporary floating value on the RX pin and bad characters was received during testing.

Solves Issue #7511

Pull request type

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

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2738
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7643/

Triggering tests

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

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2372
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7643/

[mbed-ci]

Test : SUCCESS

Build number : 2465
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7643/2465

[cmonr]

/morph mbed2-build