This looks like an old problem and the solution is available from mbed but following it I still cannot upgrade firmware.
My FRDM-K64F board debug firmware was corrupted and I followed the instructions on https://os.mbed.com/blog/entry/DAPLink-bootloader-update/ to the letter . I got OpenSDA to boot as MAINTENANCE but on the step #5 the upgrade to the DAPLink did not work: after the upgrade firmware still boots as MAINTENANCE. As a result, it is only a storage device but not recognized as a debug probe. Tried to upload OpenSDA binary from OpenSDA Serial and Debug Adapter|NXP
the same result - the board stuck in MAINTENANCE and no debug probe recognized.