scripts: runners: nrf: Fix the formatting of the old --nrf-family

The now legacy --nrf-family parameter takes the "raw" family name as
nrfjprog accepts it. But now we use an intermediate representation with
{FAMILY}_FAMILY as the format, so covert to it as required.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>