Fix NRF51, NRF52 gpio_irq_init() function

[mprse]

Summary of changes

According to the description in gpio_irq_hal_api.h file gpio_irq_init() should return 0 on success:

mbed-os/hal/gpio_irq_api.h

Line 79 in 0f4a986

* @return -1 if pin is NC, 0 otherwise

Currently, it returns 1 causing the FPGA test to fail.

Impact of changes

Migration actions required

Documentation

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Pull request type

[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

[] 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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Reviewers

@jamesbeyond @maciejbocianski @fkjagodzinski @0xc0170

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[ciarmcom]

@mprse, thank you for your changes.
@fkjagodzinski @maciejbocianski @jamesbeyond @0xc0170 @ARMmbed/mbed-os-maintainers please review.

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

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