samples: mgmt: updatehub: Fix build failure on frdm_k64f board

The CONFIG_MAX_THREAD_BYTES=3 needs to be set otherwise this error
is printed by CI.

    Too many thread objects (17)
    Increase CONFIG_MAX_THREAD_BYTES to 3

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>