net: wifi: Fix MFP when security type is not given

When security type is not given but instead MFP is given, MFP setting
will be considered as security type, this is because both are optional
and no way to distinguish them easily.

Make security type mandatory for MFP selection, this way we either
assume defaults for both security type and MFP or explicitly ask user
for both. Reword the help text to reflect this.

Signed-off-by: Krishna T <krishna.t@nordicsemi.no>