samples: net: zperf: fix `NRF_WIFI_DATA_HEAP_SIZE`

Commit e33d9ee4 incorrectly stated that:
 > Now that nRF70 by default uses zero-copy fine-tune the configuration
   to get optimal memory while getting peak throughputs.

This is incorrect since `NRF_WIFI_ZERO_COPY_TX` is only enabled by
default for the nRF54L series. Update the data heap size so that the
smaller value is only used when `NRF_WIFI_ZERO_COPY_TX` is enabled.

Signed-off-by: Jordan Yates <jordan@embeint.com>