Handle MFCustomDevice::attach() failures in CustomDevice::Add()

[elral]

Description of changes

This PR updates CustomDevice::Add() to handle initialization failures reported by MFCustomDevice::attach().

Previously, a custom device was always counted as registered after calling attach(), even if the initialization of the underlying custom device had failed. This could leave an invalid or partially initialized device in the custom device list.

With this change:

• CustomDevice::Add() checks the return value of MFCustomDevice::attach().
• customDeviceRegistered is only incremented if the custom device was initialized successfully.
• Invalid or partially initialized custom devices are no longer registered.

This change depends on the corresponding update in the separate custom device repository, where MFCustomDevice::attach() was changed to return a bool indicating whether initialization was successful.

Together, both changes ensure that only fully initialized custom devices are added to the registered device list.

[github-actions]

Board and firmware folder for this pull request:
Mobiflight-Firmware.zip