LPC55S69: fix serial_set_flow_control

[maciejbocianski]

Description (required)

Fix serial_set_flow_control implementation for LPC55S69
Skip pin setting if NC
example call causing assert: serial_set_flow_control(..., FlowControlNone, NC, NC)

Problem revealed by #10924

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

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)
[x] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Release Notes (required for feature/major PRs)

Summary of changes

Impact of changes

Migration actions required

[ciarmcom]

@maciejbocianski, thank you for your changes.
@maclobdell @ARMmbed/mbed-os-maintainers please review.

[jamesbeyond]

LGTM

[jamesbeyond]

@ARMmbed/team-nxp @mmahadevan108 , please take a look this fix

[0xc0170]

CI started meanwhile

[mbed-ci]

Test run: SUCCESS

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

[mmahadevan108]

Thank you