STM32 RTC : Prescaler macro issue

[jeromecoutant]

Description

Issue found in TICKLESS mode in #8473

All macro definition should be protected by parenthesis

Pull request type

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

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 3433
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/8493/

Triggering tests

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

[mbed-ci]

Exporter Build : SUCCESS

Build number : 3053
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/8493/

[mbed-ci]

Test : SUCCESS

Build number : 3222
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8493/3222

[0xc0170]

Rerunning jenkins cloud client job