DPP: Fix @CONF-OBJ-SEP@ parsing for multiple configs

DPP: Fix @CONF-OBJ-SEP@ parsing for multiple configs

The first call to dpp_configuration_parse_helper() was supposed to use
the separately prepared tmp string with only the first configuration
entry, but it ended up using the full string that included both
configuration entries. This could result in the first configObject
getting a mix of parameters from both entries.

Fix the parsing to use only the text before the @CONF-OBJ-SEP@ separator
for the first entry.

Fixes: 7eb06a33697f ("DPP2: Allow multiple Config Objects to be build on Configurator")
Signed-off-by: Jouni Malinen <quic_jouni@quicinc.com>