examples: blufi: fix API usage for generating dhm secret
examples: blufi: fix API usage for generating dhm secret

API usage for `mbedtls_dhm_calc_secret` was incorrect, fixed by
providing correct RNG function pointer. This behavior was changed in
mbedTLS-3.x update.

Tested BluFi provisioning with this fix.

Closes IDF-5796
Closes https://github.com/espressif/esp-idf/issues/9488

espressif/esp-idf,espressif/esp-idf,espressif/esp-idf,espressif/esp-idf