USB STM32: Don't wrap direct function calls in MBED_ASSERT

[facchinm]

Description (required)

Restore USB functionality on STM32 target if -DNDEBUG is specified as CFLAGS

Summary of change (What the change is for and why)

dab09f3 added checks on some functions in the form of MBED_ASSERT on the result.
Compiling with -DNDEBUG elides the call, thus breaking the functionality

This patch restores it, while leaving the return check if compiled with standard profile.

Documentation (Details of any document updates required)

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Pull request type (required)

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results (required)

[] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[x] Tests / results supplied as part of this PR

 #include "mbed.h"
 #include "USBSerial.h"

USBSerial ser;

int main() {
      while(true) {
      }
      return 0;
}

Compiling against 355336c with -DNDEBUG doesn't enumerate the USB peripheral.

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Reviewers (optional)

@jeromecoutant

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Release Notes (required for feature/major PRs)

Summary of changes

Impact of changes

Migration actions required

[0xc0170]

Ci started

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 1
Build artifacts

[adbridge]

This is sitting on top of #11675 currently targeting 6.0