M2351: Fix STDIO_UART error

[ccli8]

Description

This PR corrects STDIO_UART from UART_3 to UART_0. With on-board ESP8266 module connection through UART_3, program using on-board ESP8266 module would meet error with this issue.

Pull request type

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

[ccli8]

Any update?

[0xc0170]

There's focus on 5.10 release candidate PR to finalize the release. We are slowly going through CI PR. Sorry for the delay

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 3138
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7989/

Triggering tests

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

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2745
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7989/

[mbed-ci]

Test : SUCCESS

Build number : 2942
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7989/2942