tests: mqtt: Fix CONFIG_NET_BUF_DATA_SIZE assignments

These were using the older name CONFIG_NET_NBUF_DATA_SIZE (note the
extra 'N'). The symbol was renamed in commit bf964cdd ("net:
Renaming net nbuf API to net pkt API").

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>